Lawnmower blenny doesn't eat filamentous algae, what they eat is film algae. Actually some don't adapt to prepared foods very well and tend to starve out.
The green urchin is not very large. It's about 1.5" - 2". We're not talking diadema type urchins, indeed, too big for your tank. It looks like a blue tuxedo urchin but is green.
Abalones are awesome. Hardy, not too demanding. Here's a link:
http://www.wetwebmedia.com/abalones.htm
I do feed nori to my tank and suspect that the abalone probably gets some of that at night. Also sometimes I put in some chaeto for the rabbitfish and I think he also cleans up whatever the rabbit doesn't do in.