Coal Generator will accept any item being piped into it and treat it as coal (I have a quarry which outputs cobblestone to adjacent generators and that generates power).

That is actually a very funny bug but I suppose I better fix it :-)

Yeah cobblestone powered quarry is the best :D

Fixed in upcoming release

another thing, is the coal generator supposed to work like a furnace? or accept only coal? because I couldn't use alumentum.

It only accepts coal. It is a 'coal' generator after all :-)