If you keep a pair in a 10, you probably wouldn't want to risk putting anything else in - 10's are just not big tanks, and with three fish in it will be biologically unstable, and be high maintenance. If you're thinking of a tank, get the 10 - it's mch, much more stable for not much more money. You can take the 'stuff ' fom a 10, put it in a 20 , and it's just better straight away thanks to the extra water. With a 20, you could probably get one of the more placid dotty backs, or a banana wrasse in.