All I have to keep fish in IS buckets!! lol
Seriously, as long as the fish isn't too big and dirty, should be ok. When I only had 1 tank, I had a really, really mean Pseudotropheus zebra that was killing everything. He lived in a 10 gallon bucket for almost a year (I was young and really liked him, but wanted to keep my other fish).
Funny thing though...my buddy really wanted him, and one day I broke down and gave it to him, thinking I would at least give him to a better home. Died a week later...