The pinouts are very likely not the same, but they're also very likely not tremendously dissimilar. They're definitely not 16 bit - not sure what you were referring to, Trash80toHP_Mini, but if you're talking about address lines, the ROM slot would at very minimum support up to 4 megs, which would require 20 address pins (the ROMs are 32 bits wide, so 2^20 would give 1 million 32 bit words, or 4 megs).
That actually is an issue with the memory. I sell Macs as an Apple value added reseller, and I know that most 800 MHz DDR2 memory will run just fine in MacBooks, MacBook Pros, iMacs, and Mac minis, but CERTAIN 800 MHz won't, and it has nothing to do with what kind of Mac it is. I have two 2 gig DIMMs which are waiting for a machine because they (the memory) are 800 MHz only.
The problem with adding more slots is that you need to generate chip select lines based on the address space being accessed. It's possible that the massively integrated chips on the Quadra 630 type machines could support more than 256 megs, but if nobody has documentation about them and their pinout, it might be difficult or impossible to ever do this.
Just FYI, you have a two SIMM slot motherboard, so you can have two 128 meg SIMMs plus the four on the motherboard, or 260 megs.