Looks cool. Some questions/comments.
So the idea is that you have in-game favour, and between-game favor, and these have no relation to each other at all? If so, I might suggest renaming the new favor something else, like renown or something. No reason to create confusion where it's not necessary.
The main thing that leaps out at me is that I feel like in terms of model power, you are drawing some conclusions that the game as standard doesn't really support. Mainly in terms of base size and titans. I can use this quote as an example.
"Recruiting models between games:
Still working on number, but 5 for a small base, 7 for medium base and 10 for titan base."
You seem to be working with an assumption that the larger the base, the more powerful the model is. But that's not how Arena Rex is actually meant to work. If that was the case, then the core game would have balance issues I feel, since you can field three huge size bases, or three titans, against three small based models, and these all count as 3-card cohorts that are meant to be balanced against each other.
To break it down: The game has four different base sizes. The words are just ones I would use to describe them, because these are the corresponding terms in WM/H.
30mm = small
40mm = medium
50mm = large
60mm = huge
These base sizes do not indicate power. A 30mm based model (like Iocasta) is often more powerful than a 60mm based model (like Sereqet). And in terms of the game, they all "cost" the same.
Then there are model categories. Currently there are three in the game.
A model can be both a beast and a titan, or one or the other. Gladiators are all models that are neither beast nor titan.
Beasts and titans both get some special rules attached. But they are not attached to base size. As it is at the moment, all beasts and titans are either large or huge based, but in theory, you could have a small-based titan. And we do have one large based gladiator (Stheno).
Finally, you have a special model aspect called "stages". Additional cards with additional hitpoints. This is the only aspect of a model that changes how much it is "worth". So a model with two stages "costs" the same as two models with one stage.
So if you want to reference how much models are "worth" in the core game in your campaign rules, the only thing you can do is reference number of cards. So a Bestiae starting cohort, for example, would include Anum, and two cards' worth of beasts. Those cards would then either be two one-stage beasts or one two-stage beast. Since being a titan doesn't make models cost more in the core game, I don't understand why it would in your campaign rules.
I hope that makes sense.