Hi Linda
Weekly water changes are perfect
Make sure you also clean any foams in your filtration system in the outgoing changewater weekly as well. A 10g water change in a 50g system is about 20%. If it was my 50g I would probably only change about 7-8g weekly, but 10g is okay too.
Temp of the new water should be the same as that of the tank, plus the new water should be aerated. Put a 50w (or smaller) heater into your new water container as well as a small powerhead for aeration, and the new water will be just the way it needs to be.
As for new fish, I'm fond of tank raised ocellaris clownfish. They are often available at lfs. Get two and they will pair up, becoming constant companions.
HTH