Well, 'casual competition' is kind of an oxymoron because those two words are inherently contradictory. As long as you don't have any commitment to rankings or winning you are automatically casual. This makes this question a bit difficult because without commitment there is no drive to give you a more elaborate answer than "do what you want".
You seem like the type collector type of player; someone who enjoys breeding and training Pokemon for perfect IVs, EVs and optimal movesets for lulz in spite of their actual battle capabilities (shiny, perfect Phione is one of the extreme examples). In that case you probably want to commute between in-game and competitive forums but it would be easier to just ask a question. We will find out where it belongs.
Our second guess: You are just a competitive player who wants to use a certain Pokemon. In that case you are already in the correct subforum. Just ask.