Any suggestions on fish?
at this point your options are pretty open, look through fish profiles and decide which ones you like and will fit your tank. with the size of your tank a community tank with small fish would work.
You could get a couple angel fish a school of tetras and a small school of cory cats. You could do live bearers like guppies.
You could do dwarf cichlids, tetras cory cats. A small pleco could work (like bristle nose pleco) lots of choices....
should I be adding Co2?
although im not an expert im gonna guess you have low to medium light, CO2 becomes a big requirement with high light. You can try to run it without CO2 and see how things go, just dont expect spectacular growth.
Could I possibly add some dwarf hairgrass??
You can try dwarf hairgrass but im not sure how well it will do. my feeling is there are so many variables that can effect how plants do, if there is a plant you like get one, try it if it dies hoepfully you only lost a few dollars. if it works, great get more.
or maybe java moss?
java moss would grow, you could tie it to you stump and have it grow over the fake plants on the stump.