Vermillion is fun, just there is a fairly major downfall in that it's difficult to find a human to play with often.

Regarding using particular minion types versus another, if you bring up the hotbar window, you can see all the vulnerabilities. It's kind of like pokemon.

I just recently finished the vermillion log to get the twintania pet. It's not super difficult. Mostly I'd say it's just those before mentioned vulnerabilities and having a idea of what the minion's traits and special moves are. Even if you have no interest in the game whatsoever, looking at their special moves gave me some chuckles. One of the funnier ones was like Minfillia's "Hello Hello" special move were she murders people by calling them on her linkshell (it's super effective!!!). Some of the traits you wouldn't expect to be worthwhile (but are!) are speed and defense. Getting a few minions over to a enemy will often keep them occupied long enough for some of the "old razzle dazzle" to make them stop something you don't like them doing. Another thing that helps that I can't stress enough is, learn how many points minions cost if you want to use them often. My favorite cheap move is send a army of disposable and fast soldiers that didn't cost much point wise out to die for the good of their master... :x If you are feeling like a merciful master you'd send a minion with a hallowed ground like special move though (like onion knight but there are several).