Very few seastars can be spot fed. I would not guarantee that feeding rotifers would help as a general rule. Linckia and similar are not filter feeders.
However I see no problem with other brittlestars (avoid the green). No real problems with that.
Other seastars, IMO, are not a good choice. Stick with the one and hope for the best. It takes 18 months to really know how well it is doing anyway as they take upwards of a year to starve (yes, true). But as Linckia and Fromia have similar diets, based on amount of LR, I would be extremely hesitant to add another as it may just speed up the death of both.