importing MPEG 2 files of different aspect ratios

Hello, this is my first post. I'm very new to Final Cut and video editing in general but have lots of experience in recording and production audio.

I'm looking to edit together various stock footage files from archive.org for a soundtrack(music) project i'm working on. The audio has already been created.

The video filse are mostly mpeg-2 with a couple of .mov and .avi files. These file have been coded with a range of aspect ratios. I'm wondering, how people here might go about setting up their workflow. Do I choose a desination ratio from the outset, do i transcode the various file types to a single format? etc

Many thanks

Andy