I've never had a problem buying fish from different shops in the same day, I try to do that if I'm wanting a few fish of the same species just because if there is any chance of breeding there (should) be less chance of dificulties with fry whereas if they were from the same shop they could be from the same family.
Well it turns out the only decent place near me is Maidenhead Aquatics. I won't go to pets at home for fish. I went to an aquatics place yesterday in a garden centre and OMG it was awful. It was this dank dark room floor to ceiling with tanks that were stinking and dity. the water was dank and cloudy. It was roasting hot in there. There were a load of empty all cracked stinking tanks with stgnent water in them. Was awful.
How many fish you can add depends on your tank size and stocking levels. Asuming your tank is fully cycled you should add fish a few at a time and then wait a few weeks before adding more to allow the bacteria to grow to match your bio load.