Working with small size files in PP?

Hi - I wonder if you can help.  It's been a while since I used PP but I've been sent a few MP4 videos to edit together for a sequence to post on Tik Tok.  My problem is that the source videos are very small (the smallest one so far is 640 x 368 and there are others at 400 x 848).  I've been watching videos about working with multiple-size clips and the recommendation was to create sequence settings using the smallest clip size.  I've tested this and the clips are very small.  The recommended size for Tik Tok is much larger 1080 x 1920 and portrait orientation (some of the videos I have are landscape, some portrait).  I would prefer to do a landscape orientation and a lot of the clips are this, but how do I resolve the size issue please?  If I create a sequence based on 1920 x 1080, these small clips become very pixelated.  Is it feasible to create a video for Tik Tok at only 640 x 368?  Any solutions gratefully received!