Hi Peter,
Welcome to the community! In Premiere Pro, you can combine media with different resolutions & frame rates within a single sequence. You can import all your files directly into Premiere Pro, as these formats as natively supported. You may create a sequence with a resolution of 1920x1080 at 25fps, matching the frame rate of most of your clips. For lower-resolution clips, such as 720p, you can right-click on them in the timeline & select Set to Frame Size to scale them up to match the sequence resolution. After editing, if you're exporting for social media or online sharing, we recommend using H.264 format with export settings of 1920x1080 resolution, 25fps, and a high bitrate for good quality.
Please note that both sequence and export settings should ideally be based on your final delivery platform, but these settings should be a good starting point. Hope it helps. Let us know if you have any questions.
Thanks,
Sumeet