Salt is ok to use, but don't overdo it. 3 Tablespoons in 10 gallons of water is plenty. When doing the water changes, don't dose the salt again at full strength. If you're sure 50% of the water is being removed each time, then 1/2 dose (1.5 tablespoons) of salt each time is safe. Salt does not evaporate, so if you take out 40% each time and dose for 50%... it will accumulate. Too much salt can be toxic, so be careful, please. It is also safe to just pour the salt into the water, no premixing needed. If the fish eats some of the crystals, that's ok also.
The goldfish I mentioned earlier... I didn't use any salt in the water, just kept it clean long term, and made sure the fish had good, proper food, lots of silk plants to help avoid stress, and plenty of space to swim around. A good filter plus an added air stone can also help a lot. Goldfish are about the dirtiest fish there is, so if the tank is 10 gallons, its a safe bet you'll need a filter strong enough for 20 gallons.
When you get the new tank, something to keep in mind... this is a fish that should grow to about 6 - 8 inches when full grown. Full size is usually obtained by 3 - 4 yrs of age, so they grow quickly. (The one I have now is about 6 - 7 inches and its about 2 1/2 yrs old) Once the water conditions and space issues are fixed, it is likely the fish will begin growing again, and at a normal (or almost normal) rate. The bigger the next tank is the more money you'll save in the long term and the easier it will be overall to care for the fish. For 2 fancy goldfish, expect to need 90 - 120 gallons. If you start out with the larger tank, you'll save a lot of money not having to upgrade every 6 - 8 months as they grow. The same is true for the pleco... it should begin growing again and rather quickly... and a standard pleco gets about 18 inches long. 90 - 120 should be large enough to keep all 3 fish long term, provided it has good filter and maintenance.
(I'm not trying to be pushy, just trying to save you some $$ in the long term)
Good Luck to you!