Yeah, the resistor matrix configures the computer for the inputs. It'll be tagged on the computer (at least it should be) what the current configuration is, but given that you have a Collins HSI, it'll have to be reconfigured for sure to a the KI-525/KCS55 input which is what the Garmin's output.
You are correct that G3X or G5 you'd have to keep the separate CDI for the KX165. Even if you weren't interfacing with a 3rd party autopilot, the G3X/G5 don't talk analog inputs which is all the KX165 outputs. With the GI-275 it can accept the analog inputs from the KX165, so it could take your 430W and the KX165 as inputs, _and_ talk to the s-tec and handle the switching of the nav source to the autopilot.
A G3X installed for IFR, does require standby instruments, which can be met by the installation of the G5 as well. 2 GI-275's (one ADAHRS+AP, one ADAHRS) one configured as an ADI and one HSI/Standby ADI would meet all your standby requirements and could eliminate (if desired) all the 6 pack instruments, including the TC if you install the GTP 59 OAT probe, however, you need to retain the TC no matter what to drive the S-tec, but you could eliminate the rest and assuming no other need for vacuum source (boots?) you could remove your vacuum system too whichever road you went down (G3X+G5, dual G5, dual GI-275). Not sure why they quoted you an extra GI-275