You are quite right about the rails. When I was designing my machine I brought in the THK rep and he said that everyone over-estimates their needs by some considerable margin. Because of cost restraints he recommended 25mm GSR rails for my X axis which is 2200mm long. GSRs are like half a normal rail and therefore take less load. I am sure the 15mm rails will be more than enough for your needs.
Can't help on the aluminium - sorry.