Yeah I have heard that works, but when I moved my oscar into the smaller growout, I moved the smaller fish from the growout into the big tank and they are doing great. the only thing in the big tank that would eat them is the oscar.
If I was you, if you really want to try this then you should negotiate with the store owners first, tell them you're not sure if the mix will work and that you may bring the fish back in a few days if things don't work out. most stores will have like a return policy or whatever. that way youre not loosing any money and you won't lose any of the flagtails if they do get aggressive. just make sure you have somewhere to seperate them to, even buckets.
My little flagtail is peaceful to all my fish but when I put something in there that he can eat he goes hyper aggressive chasing and biting anyone who tries to take his food.
ps, I used to have a 10" female Mayan, she was so mean I couldn't keep ANYTHING with her, constant glass banging