no matter on the size, your going to get some sort of aggression. Some species are more aggressive than others and each fish has its own personality. Also, at first, the fish will set up a pecking order so don't be surprised if you do see the aggression. They might find a territory that they like which will also spark aggression. Or if you throw females into the mix, that will definatly spark aggresion.
Back to your Q. IMO it would be better to get the smaller one. I believe it would be easier on the fish that way, cause there still growing and don't have that adult aggression that some fish get. I have an adult German Red that gets mean, he didnt get that way til he reached sexual maturity. I could not imagine dumping a bunch of adult male with that aditude all together at once.
what you can also do is look at the fish temperment before you buy. Also the most colorful isnt alway the right choice. There's always a domanint fish(or a battle for it) and will mostlikly be the most colorful. Hence, if you grab all the most colorful ones, you more than likely grabbing all domanint male and your putting them into a big royal rumble for dominance.
that's my opinion on it, you never know though, you could get all adult put them in together not have much of a problem. Personally if I was stocking the tank, I would get them at about 2"-3". Watching them grow is part of the fun for me.