silver dollars get huge and need large groups.. so having a very large tank to home them in is crucial. besides that IMO i find them nippy and would most likely shred the guppies... so no.
I have read they get anywhere from 5-8 inches... most sites saying 6 inches... so... you think they wouldn't work in a 55 gallon? (Without the guppies of course. )
OK. I can see where a 4 ft tank would be a problem for darting bigger fish.... what about keyholes? Would they be OK with guppies? I've read of people keeping them together. (Hopefully.... the guppies will all be gone eventually! To good homes.)
And how many keyholes would be good in a 55? A single pair or could you do more?