It is hard to gauge the gauge of the wires. (pun intended)
The Fiero wiring uses PVC for insulation. The 3800 wiring is crosslinked TXL. So things end up changing. The same gauge wire in TXL is much smaller overall than the same gauge in PVC. The second is that the insulation can run hotter without damage so thus the wire can also.
Bottom line - If you use the same wire on the 3800 injectors that was used on the 3800 injectors that is fine. Also since the injectors do not fire at the same time, in this case splicing the 3 of each bank or even 6 of both banks together into a same sized wire is OK. If you noticed on your donor harness this is what GM did.
Two things would worry me about fire with a swap harness - 1 Wire the negative battery cable only to the engine block and not to the vehicle frame. This is different than what GM did. If the battery cable would come loose at the block but still be grounded at the frame, the small wires in the harness would then become your 'battery cable'. They could not handle the load and would become red hot.
The second is to make 100% sure you have no leaking injectors. This did happen on one of my harnesses. It was quite clear it was caused by a leaking injector since all the fire was at that portion of the harness. A shorted wire would have scorched the harness along the run of the wire. This was not the case.
[This message has been edited by phonedawgz (edited 05-17-2015).]