I have a similar setup (G430 GRT/HX and Trutrak) but can't talk to Stec specific issues. Not familiar with your installation either.
The optimal way to fly this setup is have the GRT control the autopilot using the G430 data (or GRT data for backup). In my installation I have a switch that allows me to operate that way, or to switch back to having the G430 drive the AP directly.
So a first questions are, "are you wired up so that the GRT can use the G430 data to drive the AP?" and "do you have a switch allowing you to switch back and forth?" Figuring out what mode you are in or what modes are available would be a first step.
Said another way, if you are not wired up in a way to allow the GRT to drive the AP, you would be missing some of the advantage of having added the GRT.
I think the red FPM means nothing. I noticed it during flight and couldn't figure out what it meant. As I recall the factory explained that when flying in a strong crosswind, the FPM may move off the left or right edge of the screen and when it does, it turns red to let you know that it's actual location on the synthetic vision backdrop is not to scale. I assume you are bench testing but I would suggest ignoring that.
Is your G430 a WAAS box? Do you or did you have GPSS steering? That is, did your old setup anticipate turns or did it over fly waypoints before turning?
The optimal way to connect the G430W and the GRT is with an ARINC connection. If you have that, the G430 ARINC config should be ARINC IN1 and IN2 are OFF and the ARINC Out should be LOW and GAMA429.
If you only or also have an RS232 connection, the G430 channel linking the 2 should be set to RX=SHADIN-FADC and TX=AVIATION. I don't know why but that's how I'm setup.
Hope some of that may be of help.