Your monitor should be just fine with 15k, the ArcadeVGA puts out 15k specifically because arcade monitors use 15k.
Peale is right about the wiring standard being slightly different from JAMMA, have a look at HardMVS.com- there's an article there on the differences. If I remember right, there's power running where sound should be, or somesuch. You'll definately want to fix that before you plug in your JPAC, you don't want to fry it.
As for disconnecting the power from a standard JAMMA setup before connecting, I dunno, I never used a JPAC. If it's necessary, I'd expect it to be mentioned on the Ultimarc JPAC page. If it doesn't say anything about it there, I'd assume that all you have to do is re-wire the JAMMA connector from NeoGeo to standard, and you're done.