Yes, the system could give us more details on how to calibrate such spells and powers, I agree. Can you do everything as a player when designing it? Frankly, you shouldn't. It's easier for the game master to design something without breaking the game.
What parameters we know for the base Creo Animal spell includes:
- The Magical Might of the creature cannot exceed the level of the spell. This might seem powerful, because it allows you to create a Might 50 creature... and it is. But bear in mind that Magic Might is a double-edged sword. A Might 50 dragon is likely going to want to enter magical meditation in a normal covenant aura of 5, because otherwise it will lose power over time;
- Requisites. This seems simple, but it's not that simple. If you stick to using Animal, you're stuck with a creature that has mostly physical powers, personal shape shifting into a different animal types and perhaps powers affecting other animals;
- Size. If you want to create a magical animal larger than +1, you need to throw in magnitudes;
Another important insight, I would say, comes from Covenants: "Many magi cannot create a thing unless they are aware, in general, of that thing’s nature. A magus who has never heard of an aloe or a cameleopard cannot create an aloe or a cameleopard." In other words, to create a magical creature, you need to have heard about the concept. This means investing in magic lore, tracking down legends. To create a phoenix, you need to have heard of a phoenix. This incidentally means that you can't sit down, and throw virtues together on a magic animal. If you want a dog that slays by looking into your eyes, has a metal skin and teleports... you need to hear of a dog that does that. In other words, your gamemaster and/or your troupe pretty much controls what you're allowed to create. So most likely, you're not deciding the Might of what you create... but a check with magic lore might allow you to guess the Might of a creature you would design a ritual for. At least that's how I'd handle it. And then follow the magic animal design rules in RoP:M.
For Merinita mysteries creating / conjuring faeries, I've done an attempt to propose spells that work with existing creatures here.
I'll stay out of the discussion of Faerie / Infernal / Divine powers because I really haven't used them that much, but yes, I suspect most of them work with "talk with your troupe".