New user - immediate problems with playback

Hi there, I am a recording engineer with a 2012 Mac Pro that has been maxed out with Radeon 580, 64GB RAM, Nvme drives, and upgraded CPUs. The machine is a beast. I run Logic Audio on it, and have Mojave as the OS upgraded to the latest version. I decided to purchase and install FCPX on it, so I can do some video editing.

The install went fine, but when I started running it, it complained that some of the Audio Unit plugins I have were not supported and it quarantined them. Anyway I took some video that I recorded from Skype and imported it in. It took it, looks fine, created a project and moved it to the limeline. It seems to play ok. Then I wanted to add some intro footage that came from Sony Vegas on a PC as a MP4 file, and I imported that fine. During the time of adding it to the timeline, I used the regular play back button, and the thing just goes into a "spinning wheel of death" spiral that never seems to come out.

I have been trying to work out why I just spent $300 on a lemon. I thought that maybe it is my AU collection, so I paired back the ones that I rarely use and tried again. Same thing. I thought that maybe it is the codecs used in Sony Vegas so I tried exporting to different formats incl. AVI, etc. but no difference.

I run a MOTU Midi Timepiece and MOTU 828es USB connected interface for the audio. That seems to be ok initially, but once I start to add 2nd and subsequent clips to the timeline, I'm toast.

Does anyone have any idea what I can do to try and resurrect this? There are things I simply can't do (like OS upgrades) due to driver compatibilities and this is being used in a working recording studio, so we can't disrupt the workflow of audio recording sessions. I was hoping this was going to be a simple "plug & play" experience, but clearly that didn't happen.

Anyway if anyone has any suggestions, I'm all ears.

Thanks
K

Final Cut does not support third-party audio plugins as well as Logic Pro X does. It's not a DAW.

As for that video clip, H.264 is very vague, so much else can be done to a codec like that under the hood. And Windows handles codecs differently than a Mac does. I'd take it into Compressor and convert it into ProRes 422, considering it came from a Windows PC.