The use of Pokemon Showdown is fine. I get that its a good resource and fully scripted battles are a lot harder to do; I guess it just comes down to what people choose for their teams.

However, I'm also a little concerned about the allowance of Legendary and Mythical Pokemon on our teams as well.

I know these characters are supposed to be masters and the best of the best, so having some extraordinary Pokemon can be expected like the Frontier Brain who had an Articuno or the guy from the Sinnoh league with Darkrai and Latios, but not having any restrictions seems a bit strange. At the moment there is nothing stopping me from adding Arceus (Mythical) and Mega-Rayquaza (Legendary) on my team, which is both kind of overpowered and also really weird that any trainer could capture such supremely powerful beings.

I'm not sure why you are equating Mythical Pokemon with Psuedo-legendaries either.

Aside from that, I'm wondering what the rules are on Z-moves or Mega-evolutions? Are we allowed both on the same team? Neither? Or should we just pick one or the other? I'd personally prefer the latter.