As long as you’re aware that it may look different when you export, or if that’s a certain look you’re going for, then you’re fine just editing as is. Or maybe you’ve got a bunch of clips that are 29.97 fps and a few that are 60 fps. You may want everything to be in the same frame rate, but you may not care that you have a UHD clip or two in there among a 1080p sequence. The last thing I’ll say in this section is that there are a LOT of ways you can go just in this initial stage. They definitely are constantly working on this function, but just be mindful that you may have issues with too many different files. However, if you have a lot of different files in there, it can cause some lag and, in severe cases, can cause the program to crash. Then you simply export out the settings you want and it will make everything conform automatically. Premiere, for instance, will let you add multiple file types, frame rates, and aspect ratios all in the same sequence (it will even ask if you want to change the settings to match the clip, but you’ll want to select “Keep Existing Settings”). I recommend conforming your media beforehand, as well, especially for things like demo reels or documentaries - but if you don’t care to do it before, you can use your editing software if it allows it.

You can use any video-conversion software (I use MPEG Streamclip for nearly everything), or you can do a batch export from AE, Premiere, or Final Cut and get everything to match perfectly (excluding vertical, 3D, and VR video, of course). In my experience, I’ve almost always converted everything beforehand to identical settings so that I won’t have to worry about any aberrations in my footage, or any performance issues from my editing software. The first thing you need to figure out is if you actually want to have everything match within your edited sequence (frame rate, resolution, codec, etc).

Xmedia recode how to trim a video