Howdy gents... old thread and you've likely found a solution in the meantime, but for those who are still wondering, I recently mixed and matched this very thing with amazing results. Just bought a 2026 GMC Sierra 1500 AT4 and the first thing out of the gate was to install a Rough Country leveling kit with new UCA's. Being that I had to remove the struts, I figured it would be the perfect time to also swap out the shocks all around. I went with this kit:
Rather than using the spring strut spacers that come with the RC kit, I chose to adjust the retainer clip on the Bilstein's instead. I measured from the lowest (stock height) on the Bilstein to the highest (1.1") against the thickness of the spacer and they were damn near identical and accomplish the exact same thing of pre-loading the spring. I did however, still use the lower spacers that go in-between the bottom strut and the lower control arms. This turned out to be the cleanest of all options, and it leveled my truck perfectly. In short, you can absolutely mix and match a leveling kit with the ride height adjsutable shocks, but I'd be cautious not to go too much or too high if you decide to use all spacers AND moving the retainer clip up on the shock. That might be too much pre-load and give a harsh ride, along with all the other issues like strain on the CV joints, travel, etc. Hope this helps!