PM6100 definitely needs a ROM DIMM.  There were no models with soldered down ROM.   The ROMs used to be pretty common.

I recommend that you take BeachyCove up on his offer.  Failing that, Rob Spivey, Macmetex on Ebay, had some.  I don't if he still does.  I think their Apple part number is something like 341S0757 or maybe 343S0741.  

The 7100, 8100 and 9150 also require a ROM DIMM.

The machines will power up without a ROM(that's controlled by the ADB/CUDA controller) but they won't make any boot sounds or perform any functions.

Actually, these machines will power up with the CPU completely desoldered from the logic board.