When I started mine, I added the smallest, least aggressive one I had... Which was Mew by the way, and let her swim around and explore for about 10-15 minutes, after which I floated the rest of the cups in the tank for a bit, and released them in order of least aggressive to most 2 at a time. They all flared, locked lips and bit each other constantly for the first hour or so, then they explored together and all that, while still showing some aggression. I only had one troublemaker (Daiquiri), who built a rivalry with Faye, but they've since settled their differences. lol... When I do 100% changes, I do the same thing. Based on my observations I release the least aggressive one first. It doesn't even seem to matter anymore since there's barely a pecking order anymore. If I'm introducing a new betta she's always the one to go in and explore first. Eh, some people may not agree with the way I do things, but it's evidently worked so far.
Winston~ Damian~ Twitch ~ Evans