wel, i gave this some thought.
conclusion:
combining machines is OP dont do it.giving them more slots, seems like an good idea to me
adding variable EU ussage's and efficiency's also sounds good.
overall, good idea
while true balance would be impossible the OP-ness of combo machines and tools can corrected by randomly assigning the machine flaws like reduced speed and efficiency or the tendency to explode and thats assuming you get the machine to randomly spit out a combo machine for you in the first place. a perfect combo machine would be very rare indeed