To be clear.. the only reasons 5.5 is working for you at this point are that (a) you are using a licensed cape, and (b) I haven't had the time to update xLights to move that to 6.5. Don't plan on uploading to 5.5 lasting much longer....
The Pi4 has a hardware limit on h264 encoded video of 1080p. If you want higher, you need to use h265. You could try using Handbrake to re-encode the video and see if that helps.
Alternatively, go to the outputs page in FPP, the K16 tab, and just re-save. That should reset the configuration for the smart receivers from the FPP side.
If you are getting that warning it's because you didn't enable the DDP channel input. Thus, it's not processing the incoming data at all, just displaying the warning.
This release is probably the "biggest" release we've done in a very long time with changes that have large impacts through out the code base.
The main new feature is...
Personally, I'd strongly recommend selling the Pixies on eBay and getting a real e1.31 controller. The Pixies will not work perfectly with xLights/FPP. They may work "well enough" for some cases, but there will always be additional limits.
Second: I wouldn't spend much time getting it...
I believe QLab support OSC cues. You likely can use that to trigger fpp commands via the fpp-OSC plugin. You wouldn't need separate MIDI hardware or anything connected to the FPP instance.
There are two different test modes targeting different use cases.
The main one people use is the Status -> Display Testing, but this is more targeting complete display. There are ways to select just channel ranges for various models/strings, but the information provided by that is "limitted"...
To go a step further, I personally leave "auto size" turned off for each controller and give each controller a block of channels large enough for everything it has along with a bunch of extras. For example, the controller with my arches and pixel stakes currently has about 8K channels of...
I wish the power supply was available in a 120V version... :( When I first started out many years ago, I could get various weatherproof and waterproof power supplies at 500W or so for the US market. Cannot really find them anymore.