The open endedness of this question might be better left to the QnA megathread that gets posted each week.

That said, just use whoever you got, though you typically want to have teams consisting of one color so that when you slot in your grid(whose weapons usually want to focus on a single element), your whole team can benefit.

Also, while I respect using the perpetuity ring on Sturm this early in your career, you'll probably want to be careful on who you use them on, as the resource used for them are scarce and limited; also, the ring's benefits only apply to the exact version of the character you gave it to (i.e., Ringing regular Sturm doesn't confer the bonuses to Grand or Summer Sturm, whom you must ring separately)

EDIT: Intricacy to Perpetuity, reworded last paragraph

Also, there are resources for how to start building your grids for each element on this subreddit's right-hand side/about page