I forget what size tank you have but unless you have a 150g+ tank for a full grown rhom, they are out of the question. However, they grow extraordinarily slowly, so something in in the 50-80g range could last for a good amount of time if you buy a small rhom. However, when you start buying large serras when they are less than 3-4" you really can't be 100% sure of what fish you are actually buying. Just something to be aware of.
Ery erys tend to be hit and miss in terms of activity and aggression. I wouldn't put one in your tank (somewhere around 70g iirc) as that would be a waste of tank space, and I wouldn't attempt to keep those kind of fish together unless you have large qt tanks laying around.
Tbh, if it's sheer aggression and entertainment you are looking for, larger american and african cichlids would probably be a far better idea. Serras tend to be rather boring fish along with most wolf fish.
I would suggest that you try and search before asking questions. Most of your questions can be answered easily by a quick google search or a search of other forums. I'm not trying to be condescending, I'm just saying you can try and answer your own questions too. Frank at OPEFE and his site are great resources for piranhas.