If you really don't want to have more neons, then i would not get them. The existing 4 have been together for several years, and in this situation I always think it best to let nature take its course and leave them. Give them a suitable environment of course, but beyond that let them finish their time together. Adding more might
cause issues after so long, even with characins, though not likely. I have some characin species that are down to 3 due to gradual demise over several years, and some I can't get locally and one I also don't want more of, so this is my method.
As long as the neons are still around, i would not raise the temp above 77F, but there are several other peaceful fish that could be added.
On the water hardness, those numbers equate to approximately 7 dGH and 7 dKH which is not at all bad. If you want to go with soft water fish, this is very workable. Were it me, I would work on reducing the pH a bit, aiming to have it just below 7 (in the 6's range). Partial dilution of the tap water with pure water will achieve this easily. I would do it slowly in stages so as not to shock the existing fish. Here in the Pacific Northwest and SW BC, rainwater is plentiful and works very well. For the minimal amounts you would need, a clean large tub container such as a new rinsed-out garbage bin outside in a good rain would collect enough. Let the rain fall for a bit before collecting. As long as there is no heavy industry nearby, it should be safe.