This plays on the idea that to be a more successful trainer you need to have a sponsor, such as a prof, a good portion of your journey is dependant on what your sponsor can provide, like a ranch for your pokemon, this can affect the amount of pokemon you can catch. The type of research the sponsor is performing, which can dictate the type of pokemon you can catch.

Though, evolution stones aren't the only type, things like Applin, Charcadet, etc also count due to evolving from outside stimuli

This idea is based on your sponsor being a researcher on stone evolutions, meaning you can only catch and keep pokemon that can or have evolved via a Evolution Stone.

The Starters would be the Elemental Monkeys.

The region doesn't entirely matter, but certain regions wouldn't work, like Paldea, as the academy is a easy to get Sponsor, which gives the trainer basically infinite resources.

The professor has the resources to give you Evolution stones, but you have to write a report on the pokemon you have after a week of having it and training it, behavior, skills, preferred food, etc. And how it may change after it evolving.

The Professor can only provide room for 17 pokemon at his lab.

To help provide profit to your professor, you send 10% of your money you've made every 2 weeks, along with most money making items, such as pearls, gold nuggets, etc.