I have both the Alpines and the Kickers in my car - and they work fine together but I have my treble set to either -3 or -4 in the head unit. I thought about that warning and ignored it and my thinking was this: The stock tweeter was 4 ohm and so is the Kicker. The Alpines are 4 ohm and so were the stock door speakers. I've been using this combo for 6 months with no problems.
First I had the alpines with the stock tweeters. The sound was better, but the soundstage was dragged down because of the alpine tweeters. It didn't go to the floor, but it was noticeable. I found that the highs still didn't sound crisp and dynamic so I added the kicker tweeters afterwards. When I installed the kicker tweeters I did a listening comparison of Alpine + stock tweeter vs Alpine + kicker tweeters and much preferred the Alpine + Kicker.
Notably, I have never heard the combination of Kicker tweeters with stock door speakers, or with the Rockford Fosgate upgrade. People with the RF (which isn't a coaxial) seem to like the Kicker tweeter combination since the stock tweeter is so lacking.
Since you have the Onyx, have you considered adding the sound deadening from the Limited and Touring trims? If I had known about them before my install, I would have put them in.
In this DIY OVERDRIVE video, I remove the driver's side door panel from my 2020 Subaru Outback Touring XT! Why you ask? Why not?! No really, mostly for curiosity for a potential future speaker upgrade, Dynamat installation, etc. After searching YouTube to find a video that covers these...
www.subaruoutback.org