How many sea urchins can I sustain in my Nano

I currently have a Sea Urchin - pinkish sort of thing not sure of the
exact species or name - it was described as a pin cushion urchin....it
trundles around the tank picking up all sorts of rubbish on it's
spines....including the odd hermit crab hitch-hiker that takes a free
ride before dropping off....

Now, the LFS has some really great looking Long Spine Sea Urchins -
totally black in colour and I want one.....in the next few months I am
getting a bigger tank 96 IMP gall instead of my little 15 IMP gall
Nano....I have plenty of algae to sustain a couple of sea urchins short
term I'm sure....but are there any other reasons you can think of why I
shouldn't go out and buy one of these things? From experience if you
don't buy when you see it vanishes for months from the shop......

Gill