I have a black "brittle" star, he is fine, never comes out during the day though. I have read to stay away from the green brittle stars, because they can be pred
atory, not sure why the color makes a difference, but that is what I have heard.
I
had a red "serpent" star, but I took him out because he attacked and ate a couple inhabitants(clown, fire shrimp, a neon goby(part of a mated pair, so that really ticked me off) and I think a skunk cleaner shrimp) that he shouldn't have. I was told that they "probably" were dead before he got to them and was just doing his job, so unfortunately I gave him the benefit of the doubt too long, but every one of them was perfectly healthy the night before he got them and when I checked the tank in the morning, each time, he was puffed up big.
As far as the stars fighting, never once had a problem, and at one point I had a banded serpent, the red and the black all together with no issues.
HTH