I have a 14 Gallon Biocube.
I highly recommend upgrading the lighting to T5HO. I use a daylight and a 50/50. No pure actinics = more par for corals. The PC lighting that came with the biocube I have over the back for a little nutrient export (in the middle chamber) with cheato growing.
For livestock. You said you already have a clown and ...firetail (do you mean firefish) or do you have a FireTail Dottyback? The firefish should be okay, but I wouldn't keep that dottyback in a 14 gallon.
You've a pistol but no pistol fish? I would highly recommend getting a shrimp goby of some kind. I have an orange spotted goby in mine. Great shrimp goby-he's in my avatar. Other options would be a highfin, randalls, ect...
You sound new to the hobby, so I would hold off on the nem. Something to worry about: In a small biocube, if the nem wants to "wonder"...well things get killed/stung along the way. Especially if the lighting isn't high enough to suite its needs.
Maybe one more nano goby after that (but no more): elactinus</em> genus or trimma</em>. Think: Green banded gobies, neon gobies, red head gobies, trimma gobies, tiger gobies ect... Just say no to Catalinas. I don't know why fish stores still carry them. Temperate water systems only. They are from the Pacific. Also stay away from Margaretta snails. Same deal.