Forgot to mention that I used commercial lobster tanks.
Keeping cold stars and urchins would mean alot of feeding and maintenance. Urchins will reabsorb their gametes if feedings are late or missed. They will also cannibalize each other. Urchins feed on kelp and broad-leaf algaes. In aquaria, they can be fed on potatoes, carrots, green beans, and peas. That's where the maintenance comes into play. In public aquaria, urchins are indicator animals in that their suffering or death indicates that the tank parameters are starting to fail. In small aquaria, a single urchin death can spell disaster for the entire population since it's hard to recognize a dead urchin before it has a chance to decay and poison the tank. If an ailing urchin dies from water quality, not starvation, the death knell will likely trigger a spawning episode and the urchin will release it's eggs or milt which will also cause chemistry spikes in small aquaria.
Starfish are less problematical. Although whether healthy or sick, stars will tunnel into the substrate. The problem with that is that dying stars will die out of sight and cause surprise ammonia spikes.