I have a red bali (is that a linkia?) that has been in my 20 gallon for 3 months or so now I see him moving around all the time just grazing on whatever he can find. I also have a pin cushion urchin in the same tank I have seen them right next to each other not giving a hoot. So at least in my case, there seems to be no compatibility issues. I also have a brittle, but it just hides in a cave all the time and only creeps out at night. I think I've seen it's body one time since I got it over 6 months ago.
One thing I tend to do for my grazers is leave the back of the tank glass fairly covered in algae. I see them (bali included) on that back wall quite often.