the long spined urchin will get huge. probably the size of a volleyball. i don't think the two urchins will bother each other, but i'm not sure it's the right choice for you. when i posted initially, i didn't pay attention to your tank size.
i guess if you want to use it for a few weeks and then return it, it might work out for you, but ultimately, if you have an algae problem, you aren't going to solve the problem with an algae eater. you need to lower the nutrient level in your tank by doing water changes and cutting back on feeding.