small fry:
I'm sorry i can't help you much with the price differece. But when i use to have rbps i got them when they were about an inch and a half and grew them out till about 6 to 7 inches. Usually you don't wanna keep anything in your tank that is smaller than the main predator in your tank. So if you got neon tetras they'd just be little snacks for them. i tried it and it didn't work.
Also with the filter you're obviously going to need something bigger than the filter for your 15ga. If you go to you LFS they shoulod be able to help you out. You also might want to invest in a protein skimmer as feeding live fish creates a surface scum. P's are messy eaters so you're gonna have to get a good filter.
Feeding i use to feed my 3 p's once a day. I would give them two goldfish between the 3 of them. But there are a lot of ppl who are anti-feeder coz of disease and things like that so its up to you. Other than that i use to feed my p\s beef heart, beef, chicken, market fish, bloodworms, squid. Usually just once a day. I also kept a crayfish in there to pick up and scraps my p\s left out. eventually he lost half his face.
Hope this helps..