IMO, and ofc opinions differ, 5 is the minimum gang you can really put something together that mirrors what pvp is supposed to be. It allows you to have roles in a way a three man doesn't.
I'd prefer a corp vs corp fight because rivalry creates interest.
I do hear what your saying about the numbers, what i would suggest is you have a 1 on 1 and 3 on 3 with t2 stuff perhaps at assault level down. This would give everyone and every corp a chance for glory. Run a 5 vs 5 bringing whatever you want (limited by a points system).
I'd also suggest that this is done on the test server - it means you dont have to worry about losses for the corps that cant aford it and the ep people have there would be equal making it skill based and not weighted toward the veterans.
Proverbs 23:20-21 warns us, “Do not join those who drink too much wine or gorge themselves on meat, for drunkards and gluttons become poor, and drowsiness clothes them in rags."