I'm using Ars Forth Edition for this for page references, as I don't have 5th - but the spells and principals hold for fifth as far as I know.
Making a full sized zoo is much too big, let's make a mini-zoo!
Beast of Outlandish Size (p 109) is the key spell for this, it's level 15 and, helpfully has a duration of Inst when Vis is used, so permanent tiny animals!
This does mean that, however, for each animal it's going to be 3 pawns of vis to permanently shrink them, a little customisation of the spell may be also required as Size directly effects a creatures health (the spell notes that smaller creatures "look like runts unfit to live" and what we want it for is to reduce the creatures actual physical size, but this is the closest stock spell there is and it's not a huge leap to do so.
There is also a limit of -2 reduction to the creatures size with this spell so depending on the Storyteller you may need a little more than that to get your tiny animals an extra 5 levels (and therefore more Vis) should probably cover us down to -4 size for insanely teeny beasties.
How much do they shrink?
Ars itself doesn't really note (in Forth, that I can find) how size changes the size of a creature; but lets check some example creatures:
Horse Size +2
Human Average +0
Wolf Size -1
Dog Size -2
Bird of Prey -3
So we can see that as far as size goes, it's not really a brilliant stat for measuring it; What I would suggest from the sizes above for shrinking creatures is that for every size change down you halve the size of the creature. So a -2 would get you 1/4 sized creatures, with that in mind let's take the level 20 spell proposed above as our benchmark and say from that we can get creatures 1/16 of their normal size - that's pretty manageable, but more Vis cost.
So the cost is going to be 3 or 4 pawns of vis per animal (depending on the Storyteller and the spell used)
But with that and your 1/16 sized creatures, they're now (depending on what they are) hopefully a lot more dangerous. If they're still too dangerous, what can you do?
This is tricky, as high might animals are going to be tough; the only comparable spell is from (Form+Technique) Perdo Vim (the spell being Demon's Eternal Oblivion p 158 that reduces a demons might) But we don't want to store demons (we'll get in trouble) we want to reduce the might of animals. However the description of Perdo Vim states:
Gen: Reduce a targets Might by one point for every point by which the
level of the Vim spell + a quality die exceeds its Might. Different
spells are required for creatures tied to different realms.
So we have a way of reducing the creatures might! This will require a custom spell for each classification of creatures (and don't let the Order of Hermes catch you trying to stock your zoo with demons!) But Demon's Eternal Oblivion has a duration of Inst and no Vim cost! The spell even notes that repeated castings of this spell are particularly effective. So we can blast our creature down to zero might with this. Note that at zero might demons are banished, so magical creatures at might zero may well snuff it, so reduce them to might 1 instead.
So now you have teeny tiny slightly magical creatures!
- What setup? A couple of large rooms will probably fit them all in, depending on shrinkage, containment is much easier with simple bars of standard metal, the creatures will be too feeble to break them now (hopefully ;) )
- How does this solution scale with incoming animals? 3 or 4 extra Vis for a new creature and a small amount extra room.
- How does this solution provide for animal feeding? It doesn't but now all the creatures are super tiny the quanity of food required is massively less (say 1/16th) less so feeding 35 creatures is no worse than feeding (say) 4 horses, depending on specialist diet. Making any specialist food is going to cost you Vis however, so that's not brilliant.
- Does this solution function in an Aegis? The duration of shrinkage is Inst with Vis, so there's no residual magic to dispel, the spell effect "persists as a mundane thing" (Durations, p 104) - so it can't be dispelled. So that's okay. But how can we get these magical creatures in? Their might is going to be 1, so there's no way they're getting past Aegis of the Hearth (Unless your Circle of Magi are unbelievable wusses!!) We are saved by the clause:
Magical, Infernal, and faerie creatures and persons may be brought
within the Aegis, if they are formally invited by one who participated
in it's casting and given a token to represent the invitation. (The
tokens are used during the casting of the ritual, and thus conform to
the laws of sympathy)
Can you say "collars of invitation" for the creatures? Sorted! Just make some whenever Aegis of the Hearth is cast.
- How much time and vis does it cost? Vis isn't a big cost here, 3/4 per creature to shrink the, Time wise the shrink spell is a level 15/20 - that's really not a stretch, building micro cages for the animals really won't be that hard either, if you can't be bothered to hire someone for it just Muto Terram yourself some cages but that's going to cost you more Vis. The reduction of Might spell is a little higher (up to 35) depending on the creature you're trying to de-power, and you'll need to recast it every once in a while to keep those beasties from getting loose.