tropical fish - mollys?

I have a 30 gallon tropical tank with comunity fish in it - mollys, a spotted dojo, some tetras. My mollys have been giving me issues, they are the only fish who have gotten sick or died at all. I swore I wouldn't get any more, but then my last girl died off… all I have now is a boy and a little baby (female). Should I get a few more females for him to keep company? Will it really be upsetting for him to not have any other adult mollys around?
Also, if I do go ahead and get a few more female mollys I'd like to protect the rest of the tank from them for a while to make sure they're nto bringing any diseases into the tank. Any suggestions as to how long I should keep them in the second smaller tank before introducing them to the rest of the group?