You are right. You only need one Lvl 80 Bloomed Girl for the Scaling. But it's beneficial to have 5 Bloomed Girls because High Lvl players with strong Rainbow Squads are more likely to toss you Friend Requests. On DMM it might not be an issue though. DMM Players are more likely to accept New Players on there Friendlist compared to Nutaku Players (Forumers are an exception).
I would bloom girls who share a theme (Dodge, Debuff, Counter etc.) first for synergy reasons.
For example Miko Anemone and Waterlily both share the Dodge theme. If you want a 5* Dodge Squad it isn't a bad idea to bloom them first.
It depends what your plans are though. Since i don't know what your prefered playstyle is, it is hard to recommend specific girls.
Ultimately it comes down to: What teams you wanna build and if you consider a girl a waifu