I have seen people sucessfully keeping acro's under T5 lighting so it is possible. But there are other factors to consider. A good reflector for the T5 bulb is key (Teklights have good reflectors) along with good water quality and flow.
What size is the tank? You say the acro's will be 10-18" under the fixture, is that all through water or part water and part air?
IMO 18" of water may be too much for only four 39 watt bulbs, but with good water quality and good flow it may be ok.
"Simply put, you believe that things or people make you unhappy, but this is not accurate. You make yourself unhappy." Wayne Dyer