How to keep native aspect ratios when exporting from Premier pro cc 2017

Hello all,

I'm running premier pro cc 2017 on a PC and have a question regarding exporting a video which contains 2 vintage videos (from the 50's) ripped from a DVD with ratio 720x536. This export also contains videos shot on iphone and HD video shot from a canon SL1. I'm exporting using AVCHD 1080i 1920x1080. A friend mentioned he would recommend keeping the native aspect ratios of the clips in order to get 1080i quality but not force the older smaller vids into 1080.

I'm wondering if there is a way to do this using premier export. I see a "change output size to match source" under output in the export setting but not sure if that's it?

Also, when I import these older vids should I be keeping their original sequence settings, or match them to the setting of the project?

Also, i plan on playing this video from a flash drive into a samsung tv. I tested the AVCHD 1080i and all the clips look good on it as far as format. But i'm thinking maybe the quality would be better keeping their native ratios.

Thanks!