If the betta has been used to living on his own, he may react poorly to the guppies. You can test it by floating him in the 90 inside a breeder net or breeder box for a few days and see what he does when they swim by. If he doesn't seem to mind them, you can try letting him out while watching him like a hawk for a few days to ensure no fights break out.
My main concern with having him in the 90 would be about how much current and how shy the betta is. Some will get stressed by all the activity that's usually present in a large community and will just hide in the corner all day. If the current is moderate or stronger, they may get worn out more easily too. That's not to say it can't be done, just keep an eye on him for the first week and be ready to move him back to his old tank if he's having fights or appears stressed.