They're keeping it to Kanto Pokemon only for this game. That includes Alolan formes since they're technically Kanto Pokemon, just different formes of them. But yeah, they definitely confirmed you can't evolve your starter Pokemon (like in Pokemon Yellow). You'll be able to get other Pikachu and Eevee that you can evolve but one of the mechanics of this game is to have your partner Pokemon on your shoulder, not that you can't do that if you evolved them, but they want your partner to stay the same form the entire game since there are mechanics exclusive to them, such as finding hidden items. As for Eevee's evolutions, you'll only be able to evolve into it's Gen 1 evolutions. While every remake has allowed you to evolve to Eevee's most recent evolutions, this isn't exactly a remake, and even Nintendo considers it more of a spin-off.