When the A.I. sends a full 20 unit stack of Briemes against your four Quintremes, your Quintremes will generally lose, at least that is what's happened to mine.

But whenever I've built massive navies of 10 Quintremes or more, they seem unstoppable, at least until they've won so many battles that they've lost enough men to be weak.

I dont think that having a mix would help any, but it would give you more numbers from less amount of time to construct than a Quintreme, & less money/upkeep.