I've had trouble with this too. I think it might be that some abilities turn enemy ships into your faction, so maybe that counts? Using the Corsov's Boarding Action, the Advent Loyalist Titan's "I'll shoot you 'til you're on my side" guns or the Advent battle-cruisers ability to turn a foe to one of your ships might count towards you building them for yourself. The last time I got this achievement was as TEC in Diplomacy 1.0 or thereabouts and they had no abilities like this back then, so this might be the problem.
Of course you cannot use abilities that give you enemy frigates, or call in pairate reinforcements, or revieve fallen enemy ships. No Frigttes means no frigates. If you want to make sure nothing bad happens play TEC Loyalists versus easy AI. That way you can go for "Go big or go home", "no exhaust port found" and "best defense is a good offence" all in one go (Yes I did that on one of the small maps).