Convert coldwater to tropical ?

bought a 28l aquarium, let it settle , last night bought 2 comets , ive since read that i shouldnt have any goldfish in such a small tank , so i will just buy a 50w heater today and give the comets to my friend , do i need to change the water (even though they have been in there less than 24hrs), i was thinking of a combination of neon tetras , cardinal tetras , a betta and possibly rasboras , but what combinations could have have them in and what quantities , (ie could i have 6 neon , 6 cardinal , 6 rasboras and a betta all together?) - the tank has an in tank filter with sprinkler bar that sprays gently onto glass , a bubble disc and air stone , and sand as substrate i also have 2 plants in ceramic rings , and 11w pl lighting .