I think that's a perfectly fine idea. I was the one half joking in the above monster block quote that if Billy is "Red Cyclone," the others could be "Blue Cyclone," "Green Cyclone," "Purple Cyclone," etc.
The only problems are that Mary's costume is the same color as Billy's and that "Grey" isn't the most dynamic color for Eugene. And the question of calling them "Blue Shazam" instead of "Cyclone," maybe.
Also, I don't know, I miss when Freddy specifically still looked like a kid, and maybe I'd like to see some of the others stay kids too? Yeah.