Your camcorders save in a variety of formats and resolutions. Please confirm that both camcorders are set up to record your video at 1920x1080 AVCHD at the same frame rate. (I know that your Sony is capable of shooting in 4K but, if you plan to use your footage in the same project as video from your Canon, you may not want to mix the 4K with the HD.)
It's also relevant to know which version of the program you're using and on what operating system.
I'm actually using Elements 12. The Canon 5D records in .mov at 1920x1080 and FS7 has been set to AVCHD. Both cameras are filming in HD not 4K.
The operating system is Widows 7.
But is your Sony also recording 1920x1080 AVC?
The test is that, when you add a clip from each to your timeline in Expert view, you will NOT see a yellow-orange line above the clips, along the top of the timeline, until you add effects to them.
I have tried this and do not see what you describe. I used downloaded footage Sony FS7 and Canon 5 D and Premiere Elements 12/12.1 on Windows 7 64 bit. Both clips are described AVCHD. The Sony file has a .mp4 file extension and frame rate 23.97 progressive frames per second; the Canon a .mov file extension.
and frame rate of 30 progressive frames per second.
Could you clarify what you are recording as 1920 x 1080 with the Sony FS7. I see modes for XAVC- I and L. Premiere Elements 14 would support XAVC-S H.264.mp4, but I am not sure about the I and L version of XAVC-.
Do I have the right camera?
What is the file extension for the Sony files that arrive at the Premiere ElementsTimeline, MXF or something else? Were these files MXF before and you converted them to mp4 or .mov before importing them into Premiere Elements project? Or, is what is on the Premiere Elements Timeline imported without any prior conversion for the original file from the camera, perhaps files with .mp4 file extension?
For your consideration.