The player 1 & 2 controls should be wired into the JAMMA harness, so you shouldn't lose them at all. Possibly the extra buttons (the ones outside the JAMMA spec) won't be. What you can do is build yourself a harness to plug into the existing harness that screws into the terminals on the J/I-PAC. That way if you want to run your Tekken board, it'll take you no more than 2-3 minutes to unplug the harnesses from one and into the other.
You sure you want to make a 4P CP? From what I understand, most people who build them hardly use the 4P.
I read your post a couple of times then went and looked into my cabinet. I think I'm following you. I thought all the player controls went right into the JAMMA harness. Upon further inspection, I can see that are several different harnesses that group the wires together and then all join into the JAMMA harness. If I understand you correctly, I could just wire my custom CP into homemade harnesses that plug into the harnesses that plug in to the JAMMA harness. Phew!
Current setup for Tekken 2:
CP==>Harness A ==> Harness B ===>JAMMA Harness => Tekken 2 Board
My new proposed setup:
CP(P1+P2)==> Harness C (new custom) ==> Harness B ===> JAMMA Harness ==> JPAC
CP(P3+P4)==> IPAC==> JPAC
Am I on the same page as you now?
You mention that people don't use 4-player often. I would say that in my house, the potential is definitely there, however it
IS a lot of work just to add two players, especially at my first attempt at a MAME cabinet. Maybe my best bet is indeed to just get the JPAC first (even though the Tekken CP only has 4 buttons per player) and get that up and running. If the crowds around my machine get too big, perhaps I could add the IPAC for P3 + P4 in the spring or something.
Thanks so much for your help Peale. It's great to be a newbie and be able to receive help without getting belittled! Great message boards here! I will definitely be sharing my experiences for the next newbie that comes along. Perhaps I'll put together a little tutorial with photos and what I learned during the process.