My magus considers binding a Kelpie as his familiar. I would like to read your thoughts about the following questions:
The Kelpie is not a Magic Animal, but a Magic Thing. How can you befriend one or convince one to follow you into your laboratory and stay there for the season-long process of binding? Maybe you can use magic/force to coerce it bc it is not a "real" animal?
My understanding is that the Kelpie is Size +3 in its native form, but it is somehow smaller in the stallion form. It is obvious that in its native form it can be a useful lab assistant for it has several tentacles. But what if it is in its stallion form? What and how can a stallion help in the laboratory? Or can a kelpie be in some hybrid/in-between form?
The Kelpie is quite big! Can a normal laboratory be a place for it to fit inside? (Native form and stallion form also)
What do you think what would be the first impression in the mind of other magi about a magus with a kelpie familiar?
The Kelpie as a Magic Thing has few personality traits. What is your idea how the bond would influence the magus' personality?
How would the bonding change the nature of the Kelpie? What about Cunning/Intelligence? What about its personality traits?
Do you consider the ability of human speech as granted to a familiar during the bonding process?
+1. Do I understand it well that the Cord calculation is based on your whole Lab Total, and not on your "Lab Total minus Familiar Bonding Level"? So if you have a 75 ReVi Lab Total you can form the bonds with the Kelpie (Lab Total 60 needed) and you ALSO can spend 75 points to buy Cord Strenght. Right?
I will tackle only some points regarding your questions...
The relationship between a familiar and a mage is based on trust and, usually, a shared interest or passion. They need to have something in common to share. Once the binding is complete, they will share a connection that can go as far as sharing thought and feeling what the other feels. So binding a familiar through trickery or coercion is a recipe for disaster if you follow the traditional way. The familiar will sooner or later find out it was tricked. How is the mage going to cope with constant feeling of resentment, despair or even hatred ? How would he be able to trust the familiar assistance in his lab ? maybe suicide is better than permanent servitude ?
There are two caveats: maybe your mage knows some darker techniques to bind a familiar that turn it more into a slave than a companion, without risk of betrayal.
The second caveat: if it is in the nature of the future familiar to be a trickster, being out-tricked by the mage would be the source of friendship and trust, through their love of trickery.
How to befriend a Kelpie ? Look at a Kelpie's trait, and find the one where the mage and the kelpie could build a relationship upon.
The mage might have to expand his lab to accommodate his new companion. Unless he empowers the bond with Shapechange or Shrinking abilities.
Unless mage in your saga tends to be very conventional with cats and owls as familiar, they might be surprised, curious, envious, uncomfortable or won't mind... Familiar are such a personal and intimate topic, who are they to judge.
It is likely that a mage wants or even needs a way to communicate with his familiar, beyond just gesture and basic noise/grunts. It can be speech, or mental communication (page 104 empowering the bond).
Since the Kelpie is a Magical Thing, you couldn't even bind it without a Mystery (just as you can't usually bind a spirit). But assuming this is taken care of, all other criteria for binding a familiar should be followed. For example, you also need to befriend a spirit familiar (coercion is a no go). The only exception would be, perhaps, the Faerie Cords, which bind a faerie familiar by force, but this still hinges on having the Outer Mystery of Faerie Magic.
Good suggestions for actually befriending a Kelpie were already given by others.
How any creature actually helps in the lab has always been subjective. Maybe the Kelpie will help by rereading notes and double checking math, or maybe he can grab vials and tools for you with it's mouth or tentacles.
Covenants assumes that a familiar that occupies a non-trivial amount of space is an improvement (a lab virtue), requiring either a larger lab or a laboratory flaw to offset it. For example, your lab might be "normal" size, accomodate a large familiar (minor lab virtue) and be cramped (minor lab flaw). But if you are not using Covenants in your saga I wouldn't sweat it. Go for simplicity and assume it fits somehow. Or, if it bothers you, invest an effect to reduce it's size to that of a pony.
The stallion form is explicetly "impressive, but not overtly magical", so if that's all they know they will probably think the magus has bound a regular and charismatic, maybe mildly magical, horse. I imagine that for the ones who do know he bound a kelpie, they would find it new and interesting, possibly try to approach him to know how to bind Magical Things since this is clearly a mystery, but otherwise wouldn't fret. There have been weirder things in the Order.
Based on the kelpie's personality traits and habits, I'd say the magus gains a fiercer nature, and a desire to eat horse meat. Also gains an interest in swimming, despite any previous capabilities (or lack of them) in that field.
How the bound changes the familiar, physically and in personality is usually left for the player to decide, and should depend on the nature of the magus, so I don't think I can suggest much on that. Otherwise, as for a regular familiar (gains -3 int since previously had cunning, gains True Friend and a +3 personality Loyal to Magus, understands language, etc.
This is very explicit on the corebook IMO.
any familiar can understand the languages understood by its master, and can speak them if it has the relevant vocal equipment
Most animals don't have the "relevant vocal equipment". I see no reason for the Kelpie to have it either (and more relevant, it's stat block lacks the magical quality "Gift of Speech"). But consider investing the bond with an ability to grant speech (don't forget additional mags for the Size). RoP:M suggests a ReAn effect to control the Kelpie, so granting it speech would be MuAn, despite it being a Magical Thing.
Yes, the Cord depends on lab total.
Consider also that the Kelpie's might is 20, within the range of a companion character in some sagas. Thus, you might want to select additional virtues and flaws for it to be designed as a companion. If it's intended to be kept at grog level, you might instead reduce it's might and drop a few qualities.
From RoP:M, p. 130, the sidebar on Mandrake of Virtue:
Some hedge wizards know the secret of converting a mandrake root into a kind of homunculus, which they use as Hermetic magi use familiars. This knowledge is obscure, but not totally unknown within the
Order. It may require a story to find the correct magical process, or, if the storyguide prefers, she can allow a magus to simply bond the mandrake root as if it were an animal."
So canon seems to suggest no Mystery Virtue is needed, but it does require a magical process, so could be sort-of an initiation.