2 t5 will prob be ok if your using the right bulbs, I'd go with ATI blue+ and ATI aquablue special,'bot give great par value and color over standard 10k/atinic. I run a 4 bulb t5ho over my 10 gallong for sps, with ati bulbs its winderful. But ive also delt with 2 bulb systems and its still fine as long as your rock is stacked high enough. the nem will move to where it's comfortable and gets food. I'd go with bta as they are hardy and not too keen on super bright lighting to begin with (people use to keep them under 2-4 bulb t8, just not great coloring). Both of mine rest on the back side of a rock that recieved constant indirect flow. I am using LEDtric full spectrum over a 12" cube with 80 degree optics for my 2 small rbta. It's my frag display/growout tank on my nightstand. I have a 46 bow to move them to once I build my own full spectrum array over it.
Original question, IMHO you'll be fine with BTA, don't do long tentacle as they live on sand bed and your light isn't strong enough for that depth. That is if your going for a hosting nem, otherwise rock flower nems are almost as tough as aptasia.