Advice: 1.to speed up the process of starting a tank use dirty water from a friends tank.
i have done this on various tanks and all fish have survived( iput the fish in the tank only 2 days after it was set up)
2. Do frequent water changes (25 % of the water in the tank.
3. dont listen to pet shop staff as they no nothing (do your own research on the net) i have various fish together that are suppose to be kept apart and they are fine.