it can be done, simply not advised by members on reef central. I have a cloning tank of about 8 RBTA's in a 12g; the key, is good lighting, and stable water parameters. Keeping the nutrient level low is key, and weekly waterchanges are a must.
it is simply used to propagate, lit by a 150w MH, and weekly 5g water changes.
I have a 12g Display, with the 70w With a rose as well, healthy and thriving. It can be done, as long as the requirements are met, I would suggest no lighting under a 70w MH, or 96w of PC over your 12, Stock lighting will simply not be enough.
Substitute hosts are appropriate, frog spawns, hammers, and torch's are common surrogate hosts, that either adjust or don't adjust (closing up, retracting tentacles) to hosting a clown. Oftentimes, they make the adjustment just fine. I have a 24gallon with tons of frogspawn and hammer hosting a pair for over 2 years, and the coral is growing and doing well.