... The AVCHD is coming from a Canon C100 and the XAVC is coming from a Sony FS5... if there are any issues or steps I need to do to make it work with FCPX. I'm using 10.4...
AVCHD appears in Finder as a monolithic single file. It is internally a file bundle with various levels. Do not ever pull out the individual video files and import them with "leave files in place". This will produce extremely high I/O rates for no good reason and slow down the entire library, even if only a few AVCHD files are in there.
Your options are import the AVCHD using the "copy" option which re-wraps and copies the media within the library, or externally re-wrap it with EditReady and then import with leave files in place. That is what I do: www.divergentmedia.com/editready
Sony XAVC is in two main flavors, XAVC-S and XAVC-L. I am more experienced with XAVC-S. A similar situation exists whereby FCPX will normally not allow you to import XAVC-S using "leave files in place". You can copy the video files outside the original folder tree and import from there, without any known issues (so far). However it might be better to re-wrap those as well before import with leave files in place.
AVCHD is only 1080p so (except for the above "bare file" performance issue), you normally don't need proxy or optimized media to edit that with good performance. XAVC is more usually 4k, and XAVC-S (like most 4k H264 variants) can be quite sluggish unless you create proxies.
A potential issue exists when using proxies on mixed 1080p and 4k material. The proxy of 4k is 1080p which is plenty resolution for editing decisions. However the proxy of 1080p is 960 x 540, a little low. Your options are live with it, or create optimized media for everything which increases your media storage by about 6x.
I can't remember if the C100 AVCHD is interlaced or PsF. There's always a potential issue intermixing interlaced or PsF material with progressive. The behavior of FCPX has changed in recent versions regarding whether you should deinterlace that. The default project characteristics are determined by the 1st clip copied, so the behavior could be different whether the 1st clip added to the timeline is from the C100 or FS5. For cases like this I suggest overriding the default and creating the project characteristics the way you prefer.
Then always visually inspect test clips of the output using VLC with interlacing turned off to ensure the AVCHD material has not been somehow degraded by deinterlacing, whether you selected that or not.
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