Only problem with canary blennies is they have fangs and a poisonous bite... I dunno if I'd want to stick my hand in the tank with something like that in there.
Your best bet is to stick to fish that stay small... a watermelon or possum wrasse would be a nice addition and possibly can help with any flatworms that creep into the tank when you're not looking...
Like others have suggested, you're nearing the limit of your fish bioload, so you don't want to add too much... the bioload may be fine while the system is running, but if it's shut down for any extended period (like during an outage), that's when problems with being overstocked will show up (aside from territoriality issues of course).
HTH.