Im sorry to ask this but im a bit confused, between the way to do it and the efficient way for a bonisagus to have a familiar to help him during his lab work
I have found a salamander size 3 Might 15
Which ability could you advise me on it ?
I'm not really sure what you are asking...
Any magus can benefit in the Lab from his Familiar's help, adding its Int+Magic Theory as you would any other assistant. However unless the animal bound has a positive Int, which rarely happens if it has Cun to begin with, it is unlikely to be very helpful. Plus you need to have it learn Magic Theory, and interpreting RAW from RoP: Magic the Might of the animal is detrimental to the study quality, however it is a common interpretation that the Familiar Bond prevents this penalty and the animal can learn normally.
As for other help, I'd be tempted to grant any bonus Covenants might list in the Lab chapter for Enchanted Devices to also count if the Familiar has soem useful Powers or some effects enchanted into the Familiar Bond. E.g. a salamander Familiar which can create fire and heat might count as "Magical Heating" in the lab as if granted by a device.
In fact, now I think of this I really like this idea!
Shapechanging. So the aurochs-sized (Siz 3) salamander can fit into a lab. 
And a Trait Docile+3, so it is not liable to throw tantrums, expand to its true size and extinguish all the lab's fires (RoP:M p.62f).
Seriously: this familiar can become most valuable, if properly trained and housed in an appropriate environment. The Cun -2 of a Salamander from RoP:M p.62f might restrict the familiar's Int after binding, though, and thereby limit its value.
Cheers
Can I ask why? Last I checked, any creature with a cunning score would have an Intelligence score of -3 after being bound (ArM5, p. 105, "The Bound Familiar"), with both that value and the amount of magic needed to modify that intelligence score being unaffected by the cunning score in any way?
With, say, a Cun +3 of a standard specimen you have wiggle space for an Int score of the familiar candidate already before binding. But with a Cun -2 of a standard specimen this looks quite unlikely.
And - as you explained - this significantly restricts the familiar's Int after binding, doesn't it?
Cheers
Huh?
A magical creature can have an Intelligence score, completely independently of the creature's Cunning.
I don't follow your argument at all?
I'll try again.
Sauryan's salamander has a size 3, so is clearly not a standard specimen. And all the rest we may work with comes from RoP:M p.62f.
The Cun -2 of the standard specimen there "might restrict the familiar's Int after binding":
* If Sauryan's salamander also has Cun, its Int after binding is -3.
* If Sauryan tries to get his troupe to start that specific salamander with Int, that Cun -2 might weaken his arguments.
Cheers
So are you saying that if a magical creature created and statted on the basis of some sort of mundane version of it has Cunning and with a negative value even, then the likely value for Int for the magical creature is low, like -3.
But if said mundane creature was known to be clever and crafty then one could argue with the Troupe to give the magical version Int 0 ?
I kind of follow that line of reasoning. Except a magical creature could IMHO have any stats which seems fitting because it is specificially not the mundane version and can easily transcend the values of such. So if a legendary wild boar harasses the locals, plays tricks on them and is impossible to catch or hunt down then it's probably quite intelligent, regardless of how dumb boars and pigs might otherwise be.
Nearly so, but not quite.
The RoP:M p.62f Salamander of Virtue as a Beast of Virtue is already a magical creature of Cun -2, Siz -8, Might 24.
Sauryan's prospective familiar is a different version of this magical creature. It could of course be much more intelligent than the rather dense being on RoP:M p.62f - but his troupe decides that.
Cheers
A salamander of size +3 is as akin to a dragon as it is to anything, I'd argue. No reason it couldn't be hideously intelligent.