who would be a good mera, aqua girl,etc

It all depends on which route they go in which sort of movie they make first.

If it is an origin story, I want Tom Curry, Atlanna, Vulko, Orm and Mera as like the big-named heroes. And maybe make reference for the next sequence of events, being the purpled eye kid with an omen (Garth)

If it is based on any story, I want them to include Garth in some capacity and have Black Manta as the main villain. But not for the first movie. Garth is severely underrated in the comic book industry and I believe some on-screen love could help reintroduce the character to his full potential.

Supporting meaning everybody else, right?

Tom, Atlanna, Shin, Mera and Black Manta.

I would try to stay away from Atlantis right now but if they choose to use Atlantis than

Orm, Tula and Vulko as well

I don't know, but Karl Urban was rumoured for Ocean Master.

Blunt would be a good Mera (both visually and performance based)

Urban would be awesome as Orm as well.

I'd say Jessica Chastain for Mera