Bettas being aggressive is a false state ment. They are territorial and won't bother fish unless their in its territory.
Most of the time, bettas just do their own thing and ignore the other fish.
I hate how people say bettas are killers and should always live alone, that's not true. Some. Bettas are very docile and others aren't. Sometimes it just depends on the personality of the fish.
In my experience, I've had tons of luck with bettas in community's. I've kept bettas with multiple fish people said they should never be with and it worked out fine! If anything, the other fish were a threat to the betta rather than the betta being the threat.
I wouldn't keep a betta with any fish that's weak or tiny as it will surely see it as prey, but if the betta is not the top dog in the tank, he behaves
I would Definitely try it out and see what happens, since its a baby he can't do any harm. As he ages, watch him a lot and see how he reacts to the other fish. Just make sure he's not homed with fin nippers as you wouldn't want his fins to be torn.
Most bettas entitled "baby betta" like at Petco is a female anyway, and they are even more compatible than males In community's
I'd encourage you to try it and if all else fails, rehome the lil guy or get a divider for your 2.5 and house him in there