As long as it adds no extra powers to the pokemon, yeah they can be customized too, sure!

And canon OCs (pokemon trainers, Wols, etc) are A-Okay as long as they have zero relation to any canon events, sure! However for trainers, no legendary or general game breaking Pokemon allowed of course, and just for record keeping ease I'm gonna say no more than three pokemon can come along. Any other special powers or scenarios may or may not be nerfed at my discretion!