I only have branching frogspawn, but have not had any problems with them and fish.
Of course, fish that nip at fleshy corals would be a bad idea - wouldn't know if they're a problem with nippers as I've completely avoided such fish.
If in contact, I'm uncertain if different varieties would sting each other. IMO, I would count on it. These corals can in some tanks develop quite long sweeper/stinger tentacles - so I'd be sure to plan in growth and room for them to expand much more than they will initially.
I have recently gotten frags of different looking frogspawn coral, that don't appear to sting my prior variety - but a different variety of Euphyllia I would expect would sting.
If you've got a reef club nearby [or even can drive somewhere for a big frag swap] it seems like Euphyllia is often propagated [esp branching forms]. The 3 Euphyllia I have are all propagated, and I'd encourage getting one this way [esp. vs. shipping] as IMO they can damage easily shipping, and it's a great way to get a healthy coral well adapted to aquaria.
The only other thing I would suggest is going for a `deeper' tank in the front-back dimension than the 55. For aquascaping, and for space for the corals to grow from the aquascape, IMO that front->back depth is great. Given reef tanks are very much about visuals, IMO worth consideration if you can bump to a 75 ... just about the same equipment, but to me, they generally look nicer.
Good luck
