- What Pokemon are available in Kanto? Per Gen 1 or 2?
In the Wild? Most of them.
But as far as you know, Wild Pokemon cannot be rehabilitated, and thus cannot join your party. However, most of them are still available to Pact with at certain settlements as long as you tell me you're interested in them.
- What do the restrictions do, precisely? Increase AP? By a flat amount? By a percentage?
Depends on the Restriction.
For example, a Monster-race Restriction might increase attributes or HP. Fire-element restriction will do something with Fire moves. Gender restriction improves your abilities against the opposite gender.
You can even do shit like double-element restriction, double-race restriction, etc.
Be as creative as you want, but I can't tell you what the effects are until after the vote.
What benefits does Pokémon leveling up convey? Is it "just" their HP formula that changes, due to the (in general) size increase, or do they get free stats and/or traits from it as well?
Levelling Up doesn't increase size. Pact-able Pokemon are all adults, although unevolved due to staying in range of an Everstone pretty much forever.
Levelling Up reflects your *combat level*. It does increase HP and MP by 10% per level, to symbolise knowing how to:
1. partially evade hits and minimising damage better
2. controlling your mana expenditure better
Other than that, it improves your hit/dodge against lower-levelled opponents.
- How do Pokémon generally get stronger? Training? Fighting battles? Fighting dangerous battles?
Attributes are improved through evolution and Pacts ONLY.
Skills are improved through training or special events.
Level is improved through fighting, but injury may cost you the ability to take actions, shorten your lifespan, or even end your life. (although that's a cruel but possible way to get rid of a member you don't want on your party, I guess. or to reset your Restrictions.)
- Would "Pokémon with multiple evolution stages" be a valid restriction?
Yes. Again, be as creative as you want.
Worst-case scenario you end up Restricting your Domain to no qualifying Pokemon, in which case it'll be a Game Over scene, and then Oak's Classroom, and then reset the vote.