Yes Fractal makes them quite powerful thus the once off use of the ability. You would have choose when to play it.
As to Swarm vs Hive. Yes I had forgotten that and Hive would most likely be a better term.
Welcome back, Scyther! I hope you are recuperating well.
I don't quite understand how the ability works, particularly the part about "affects all". If you have 5 centurions, and you activate one card, all 5 go to 15 hp? If you play one more and activate another centurion, does the one stay at 15 and the rest go to 18?
If one dies, do the centurions who have activated the ability lose hp since there are fewer creatures or is it fixed once the ability is activate?
Also, please don't use "hive"--these are not bees. Perhaps "horde" or "teem" (not team).