I used an older version of Premiere Pro....
I know that 720p footage has to be sized up since the frame size is different than 1080. Make sure all frame rates are the same. You have to decide what size timeline you plan to use. The rule is go with what you have the most, and convert the other footage.
You can do it on the timeline if the frame rate is the same. You can use Adobe Media Encoder to convert the footage. I would test both ways to see what looks better. I think they will look the same. By converting outside Premiere, editing will be faster.
It depends where you plan to show the footage? H264 seems to look fine and everything seems to play them. Files are small enough to be put on a tablet, phone, or thumb drive. My last interview, I had all of the above with a mini speaker for the tablet. We ended up watching on the tablet. In my case, Instead of a demo reel, I had the entire footage of projects to watch. They wanted to watch complete stories and ideas. So, it worked out very well. I did have a demo reel along to watch if they just wanted to see bits and pieces.
I have not used the YouTube or Vimeo presets. I have found it crushes the footage a bit more than I like. It may be a problem on my end. I think I went with H264, and let Vimeo crush the footage for playback. Takes a bit longer, but the quality is better to me.
Really no big difference between SD and HD video works.