You certainly can find some good tanks through the classifieds and in rummage sales. Most of my tanks are used. I fill them in the driveway and let them sit for an hour to see if they have leaks. Then wash them with vinegar to get water stains off and rinse rinse rinse. (Never use soap or impure bleach of vinegar products to wash a tank). I'd replace any filter media and discard old gravel though.
For moving, put your fish in buckets with tank water and put the filter or filter media in a seperate bucket of tank water. Then move them to the new place and get the filter running as quickly as possible. The fish could stay in a bucket for a day without harm.
Some stores will take fish. What do you have in your tank? It must be very well stocked for you to think that these two platies are pushing the limits. Do you test the water?
The gouramis should be fine together as long as they have ample room for territories. Keep a good ratio of males to females. Gouramis are labyrinth fish just like bettas and each have a different personality. Be ready for a back up plan if the ones you buy are too aggressive. Look for a store that keeps the two varieties together and buy from the same tank.
As for adding water without taking it out. You need to get a test kit (liquid not strips). Ammonia is not present in a well cycled tank for long, so adding water once in a while is not dangerous. However, I don't know how stocked you are and if you are cycled and I don't know how long it takes for your bacteria colony to use up the ammonia. Adding water to fill an evaporated tank can be just as harmless in some tanks as it is deadly in others.