Well, I have already fought with one of the opponents(Eloise) before her, but I can continue fighting with her, also when I visit the House of Tiana and I want to enter to her room a message appear me saying that I get a bad feeling and maybe I could try later
So, in order:
Eloise > Julia (The girl who wants Slimeballs - she unlocks after she opens her shop) > Mindy (The girl who eats cake) > Esmerelda > Lady G > Mary (The girl who wants yogurts each day - Once she goes back to her house she'll be unlocked) > Arabel > Tiana
So you need to unlock a few more opponents first.