I have a 55 gallon 48"long.I've spread out my live rock, (60lbs) across the width of the tank. I have three firefish and they have spread out claiming their own territories and ascerting dominance. Biggest one claimed the biggest rock in the middle, and the two smaller ones claimed live rock on the ends of the tank. They do fight when they cross over territories, usually it only results in their dorsal getting torn, but for the most part thery get along fairly well, which I think I was a bit lucky.
My research before buying them suggested only one per tank, but I bought three because my lfs had a sale on 3 for 40 bucks, and the lfs employee assured me they would be better in a group. Just from my experience, if there wasn't as much live rock and enough hiding places for all three, they would fight. As long as you can provide that kind of a ecosystem for them, they should be fine, but personally, I don't think I would do three again, but it has worked out well for me. IMO, they are a fantastic starter fish, colorful, interesting, and very hardy. Extremely fast and jumpers, so make sure your tank is covered, but usually only jump when scared, and even then they usually bolt for their darthole. Not sure if different species can be mixed up.
Brittle stars in my experience are fantastic members of you CUC. I have a red and black one and they took a bit of time to warm up to the tank, but they come out more now and are all over anything that's decaying in the tank, mostly move around at night. Nor sure about the other stars that were mentioned, but some stars are predatory so be careful which ones you choose. just my two cents.