To be fair most of the higher ranked mages have their own team or work solo anyway, not like they'll necessarily get familiar to the rest of the guild's fighting techniques just by being alongside some of the others in an specific situation. I have absolutelt no complaints about having a strong opponent that shows how the stronger mages aren't necessarily infallible.
I'll address this in two parts.
First, while there might be teams there will not be any solo work. This is a group roleplay, and as such it will act like one. If at any point a mage breaks off from the group to just do their own thing, unless it's reasonable, I will remove them immediately. I'm sorry, but this isn't a "You're strong enough to just say 'fuck you' to the rest of the guild". Everyone knows each other, everyone is like a family. No if's, and's, or but's about it. No solo mages in Dragon's Cry.
Now, assuming that wasn't really what you meant, and I misinterpreted that. Yes it's possible x character hasn't actually worked alongside y character all too often. BUT. A lot of the guild members of Fairy Tail didn't always work together, but when they did, you could see how their relationships outside of fighting actually mattered in combat. Good teamwork doesn't take just experience on the combat field, as it is effected by their relationship as a whole.
Eh, go with whatever and dial it back if we prove to be shit would be my suggestion. But I also run stupidly deadly DnD encounters that I can fudge roll which I guess you guys can't really do.
While this might seem like a good idea on paper, if I throw an enemy in that has a certain amount of strength, and then out of the blue dial them back so they're weaker, it's going to look weird. People don't just randomly get weaker out of the blue. So, it's either strong, and hope you guys can handle it, or not as strong as a sort of warm up.