In BL (which is borderline), you're allowed to use everything that isn't OU or uber. Many teams actually have a ration of ~3 BL:3 UU.Thank you for the reasonable responses. I don't really are about tiers though. Most of the pokemon I like are "underused," some "border line," and others "overused." I think I'll just go to Marriland and see what I can find there, they might be more my level.
However, if you mean playing for fun literally, like using just your favorite Pokemon, you have two options. One is to pick a viable one amongst them and build a team around it. I actually grew to like a couple of Pokemon this way (Blastoise, Vespiquen, and Marowak....<3). The other is, yeah, go to Marriland, or play against your friends, because here, most people think in terms of strategy and what can best achieve you victory. I can kind of relate....my favorite Pokemon is Mewtwo, yet I would never ever touch ubers for my life D: