It didn't help that one of them came in, cloaked, and then just sat there doing nothing.
It's entirely possible that you failed simply because of this.
I've run the fleet action under the same situation, although admittedly I had two commanders and two lt. commanders, and completed it fine. (Granted it was Gorn and not Romulans.)
I did have to throw out some heals and support for the others but since I can solo half of a wave with no problems and the starbase was tier 4/5 we simply took each wave one half at a time and let the starbase tank the other half for the minute it took to kill the first half.
On the other hand, I've failed this when there were two other VA's and two Lieutenants because one of the VA's afk'd and the other kept dying to massed Klingon fire.