How can I mix a video in Full HD (1920 x 1080) with a vid in HD (1280 x 720)
At Premier Pro, see Effect control tab on the top left, select scale. Then you adjust the percentage of size of video match your sequences setting.
If your sequence setting are HD 1280×720 then you 0r FHD 1920×1080p videos all need to scale smaller. Final of your video will be 1280×720. Same to if your sequence setting in FHD then your HD video need to be scale up enlargement, it will blurer the videos, need to add smart sharpen at Effect tab below left to an adjustment layer .