The clown pair and small goby will work.  It can't be a mandarin though.  Flame Hawk won't work - it's a carnivore and there is not enough food.  Here's what I have in my nano's

15 gallon - six line wrasse, bicolor blenny, glack clown, shrimp

12 gallon - two baby tomato clowns, goby, corris wrasse, two green chromis's (looks like a lot for a 12 g)

12 gallon at work - six line wrasse, percula clown, peppermint shrimp.

Hope this helps.  You are welcome to come see them anytime.  

I have a 12g nano - its been running for 5-6 months now.  It has about 10 lbs live rock, lots of live sand, and the following livestock:

juvenile sixline wrasse, two baby tomato clowns, hi fin red stripped shrimp gobie (stonogobiops nematode), crazy peppermint shrimp, two emerald crabs, usual snails/hermits, hosted BTA (small one) and various polyps and xenia

Here's how the community works so far: the two clowns fight constantly, so one stays in the aneomone and the other hides in back.  The goby only comes out to feed and is always tucked away in the rock cave somewhere, hiding.  The shrimp is getting huge, but also stays tucked away unless someone is near the tank - at which time it comes out to beg for food.  I never ever see the emerald crabs.  I also have two powerheads going (not full blast, but giving a soft current).  The BTA seems very happy and has planted itself in a nice spot.  (You know a BTA is happy when it stops wandering and has found a spot it likes.)  If I'd had it to do over again I would not have put the goby in there.  He's a great fish, but at feeding time, it just gets out-competed too often (e.g. its difficult to feed without over-feeding the whole tank).  Its not a very smart fish.

Problems:  one imminent and one far off.  The imminent one is bioload.  I've noticed if I don't do my weekly 25-30% water change religiously, the tank gets pretty skanky (that's a technical term, btw).  Far off problems include having to find new homes once the silly wrasse and mean clown get too big, which they will, eventually.  But I knew that going in.  All these fish can get to be 3-4 inches long each, totalling over 12 inches, not including the shrimp.  That's just way too crowded for a 12g JBJ cube, especially considering there isn't really 12g of water in there with all the rock, etc.

There are two lines of thought here.  The first is that you should never buy a fish and place it in an environment you know it will grow out of because its just too much stress for the fish and its not a healthy way to go about the hobby.  The second is that as long as the lfs will continue to take livestock back, what's to stop you from buying baby fish and growing them out in your tank? 

If you love doing water changes and can justify in your mind the "give it back to the shop" philosophy, then go for it.  I know nothing about flame hawks.  But the rest should work.