Does Fives need potency mods?

Yes

For t7 rancor should get his potency up in the 60% or better range to land it reliably. For arena it depends if your opponents mod for tenacity or not but for most debuffs 30-40% potency is good enough.

I'll be contrarian here, but I say no. He gets so many attacks with his counter, that they'll stick eventually. Mine is at 7.5% and I've never thought about upping it.

I will also add if mainly want to use for raids don't omega his basic. Saves giving the rancor or captain 5% TM.

Thanks. This was exactly the kind of info I wanted

Don't make Fives too fast for raids. His doubleshot gives much tm to the rancor.