You have to be careful with that kind of approach. You really can't put a bunch of BJTs in parallel because they have a postive temp co. So as the temp rises it passes more current, effectively destroying itself. You can however parallel a bunch of FETs, but that is normally not required -- the manufacturer has done it for you. When you trawl IRFs catalog, you see lots of FETS with super high current capability and ultra low RDSon. What they have done is put a bunch of FETs in parallel inside their package. So you just buy one and the work is done for you. I would use FETs on this, again the RDSon is so low, that you could probably even run them with no heatsink in a small SMT package.