I have a question regarding the "telekinee" spells in ReTe like "Unseen Arm", ReTe5. Base is 2: move dirt.
If a mage want to move dirt, it is this spell
if a mage want to move a leather bag, it is with An requisit
But what is it, if the mage want to move a gold coin? Te as casting requisit (which change nothing). Or does he need to cast higher spell - ReTe 15 for move metal? which would be strange - because then ReTe would easier to move non terram things as terram ones.
This - as written on ArM5 p.155 Rego Terram Guidelines.
EDIT: base 2 of Unseen Arm might already include stone and glass.
This is just one of those annoying discrepancies between legacy spells from the exemplar system of 2/3ed and the current guidelines of 4/5ed. In some cases, the authors have taken quite some care to avoid discrepancies, but in others, like this, they have done little. I agree with OP that the rules are inconsistent.
IMO the best interpretation is that there is a missing guideline for Terram as a generic form. Requiring L15 to move a gemstone does not make much sense, since you can already move a pouch of gemstones using an Animàl/Herbam requisite at L5.
I think one just has to infer the missing guideline from the core rule spells. The alternative is quite substantial revision.
No, you do not need a higher-level spell to move metal. As can be seen in the description of Unseen Arm, that spell can move a pouch of coins. There was some inconsistency in writing spells later on, but as far as I'm aware, all such inconsistency has been fixed in the errata to put them in line with Unseen Arm and not require extra magnitudes.
As for requisites, TME goes into this when examining instant transportation. Much of that can be up to the troupe. Typically if nearly all of the material falls into one category, you can just use that one Form, but if a substantial amount is of another Form you should use a casting requisite. For example, Unseen Arm to move a cup full of wine would usually call for an Aq casting requisite since so much of the material being moved is liquid.
I'd also suggest that instead of changing the level of the spell needed to move precious metals or gemstone (through a different guideline), in this instance we could just reduced the amount of material that the spell can move. So Unseen Arm can move a gold coin, but something larger (such as a sword) made from gold would be over its capability.
What is odd, is that metals and gem stone is penalised twice, in general. The size guidelines for Terram is already smaller for metals and gems. Additionally, the ReTe guidelines increases the required magnitude. This is rather bizarre.
I suggested inferring a guideline in order not to change the level, so on that point we agree. The problem is the existing guidelines which are explicitly higher even for reduced size.
And there is gradation between the techniques, because while Muto, Perdo and Rego see an incrementation of 1 magnitude for stone/glass and 2 magnitudes for metals/gemstones, Creo has an even steeper increase (2 magnitudes for stone/glass, 4 magnitudes for base metals, 6 for precious metals and 8 for gemstones). Intellego makes no distinction.
I would note that Unseen Arm only moves things slowly, so that appears to give it some leeway in the quality of materials that can be moved.
We could always rationalize it by saying that spell was achieved through a particularly beneficial experimentation. It is somewhat weak as an explanation, but not outlandish.
That's the kind of implicit guideline I am talking about. Base 2 slowly moving arbitrary materials, subject to casting requisites and normal size modifiers.
txs to all for the input 
Several interesting arguments.
I think I would rule that the Unseen Arm merely modifies the spatial co-ordinates of a solid object.
It avoids modifying and manipulating the shape and material of the target object (eg drawing a wire out of a gold coin), hence shape and material modifier magnitudes should be ignored.
There does remain a possibility that Individual size modifiers might still be applied. Discuss with troupe?
Generally agree with Ivgreen on is one.
That said, my players have certainly tried to argue, 'But my magus considers those tree faeries to be people, so can I heal them with Corpus?'
Yes, the rules have been consistent (post errata) with what lvgreen said. Movement does not require more magnitudes for metal/gems/etc. But, for example, ReTe craft magic to work metal does require extra magnitudes for metal (e.g The Armorer's Art, The Spell of Wrought Iron, The Phantom Blacksmith).
Depends on the movement speed, apparently. Sometimes it is obscured by including it directly in the Base. For example, Wielding the Invisible Sling uses Base 4. There is no Base 4 in the ReTe guidelines on ArM5 p.155, so we can infer that this is in fact "Base 3, +1 to affect stone".
How do you infer that when Base 2 can move metal (see Unseen Arm and plenty more spells)? Consider that higher speed needs more magnitudes, too. Now look at The Crystal Dart which does the same +5 damage (Has that reached the errata yet?) as Wielding the Invisible Sling but with Base 3 or lower. The big difference? The Crystal Dart uses a sharpened crystal rather than any old rock, just like Dagger of Ice (HoH:S p.35). The random rock is thrown harder to do as much damage as the slower, sharper rock. To be thrown harder, it needs at least that one more magnitude.
I'm just saying that no, the rules are not consistent. The errata you mention has not been published yet -- I just checked to be sure.
Edit: Furthermore, what Ivgreen mentions is not how the guidelines work, IMHO.
That is a little different: Unseen Arm can move "a small pouch of coins". You can easily read this as moving the pouch - and its contents not making the spell any harder.
We have no errata for the relevant ArM5 Rego Terram Guidelines or Unseen Arm. You can for your troupe derive own changes for such guidelines from TME p.107 New Guidelines for Instant Teleportation - but AFAICS none are written down.
Sure, none for the guidelines and none for that spell, as they don't need fixing. Why would we even look at either of those for errata?
Look instead at the ones that needed fixing. Did you read the erratum for Vessel of the Clouds, which is extremely explicit on this point? How about for The Prodigal Returns?
Fine, how about The Prodigal Returns? It's base 3, but there we have metal which would otherwise need more than stone. There are more examples.
However, if the majority of mass moved is metal rather than fabric/leather, then you would need the Terram part to be handled properly. Otherwise a lot of things fall apart (wrap a difficult thing inside a thin easy thing to move it with an easier spell).
Can't place these spelss. Where are they to be found?
The Prodigal Returns is HP p.90, Vessel of the Clouds is TME p.112. They were later errataed: but both are also about fast and safe flight, not the slow movement of Unseen Arm. Both errata removed an adjustment of +2 magnitudes for affecting metal. @callen did not explain, how these relate to the Unseen Arm of the OP or to your last post.