Can only agree with divegirl Puss- bet it was spotty oik of a teen who told you that info at PAH? ;-)
If you've no room for another tank, then you've no room for another tank, nowt can be done. Sounds like they're are doing swimmingly (see what i did there?) anyway. As long as they get regular water changes, which you do anyway, they'll be ok.
I'm getting another Betta fish, bought a new tank for it today- wait a few weeks for water to cycle then one of these beauties will be mine- yipee!
http://goldfishwiki.com/images/fish/Betta1.jpg