well first off, the guppy does not need brackish water. mollies have been kept in freshwater so long that less than 10% actually need the brackish, they are perfectly healthy and happy in freshwater, but not guppies, they're only freshwater.
also, yes, i'd definitely get a couple more guppies. if you have a male now and you don't want fry, just get two more males. i have three males in my 10 gallon, and they're all perfectly healthy and happy. if you get females, you need at least 3 to keep aggression down, and you are almost guaranteed to have fry.
as far as your quarantine issue.....you can very easily set up a quick and dirty
QT, i'll find the thread for you