There is no official site for middle cup that I could find, seeing as it has a small player base. But most people tend to follow by these rules from what I've read:
-All Pokemon must be Level 50
-All Pokemon must have evolved from something (even if from a Baby)
- All Pokemon must be able to evolve into something
There really isn't much of a banlist, as I said before, it's not the most popular, so strategies, patterns, niches, and things of that sort haven't been incorporated into the metagame of the middle cup.