How peaceful would you say these fish are? They are a family of the Piranha, are they fin nippers? I had a school of harlequin (15) and the most dominant one killed all the others, should I be expecting something similar here?

I plan on getting around 50 for a 55 gallon high tech planted tank along with Otos and corydoras (possibly rummynoses) How do the rummys compare to the cardinals?.

cardinal and rummynose tetras are both very peaceful fish. They can be housed with angels & discus and shoould be fine with pretty much everything else. If they are kept in small groups they might get bored but this can be prevented by keeping them in schools of at least 10, preferably more.

The same could be said for harlequins but its not what i experienced.

I'll give it a go, guppies compatible?

yeah it's a bit bizarre about the harlequin, they are normally very peaceful.
Guppies should be fine with the tetras but you will have to watch them just in case. Sometimes if the fish are in a small overcrowded tank, or stressed out by something, they might have a go at slow moving fish with long flowing fins. But they should be fine in a 50g tank. Just try one or two male guppies and see how they react.

you should'nt have any problems with cardinal tetra's. I kept a shoal and they were never aggressive and look stunning in large shoals.