If the projects aren’t overly complex import each file into a new project. You can drag sequences from the Project panel into a new timeline and Premiere Pro will treat the sequences as though they were clips of footage.
If the projects are relatively complex and you don’t want to bog down your system, export intermediate files from each project (in entirety or just the clips you want to use). The preferred choice of codec would be a 10-bit, all I-frame, 4:2:2 or 4:4:4 format such as ProRes, DNxHD, AVC-Intra, Cineform, V210, or DPX+WAV.
The difference between 24 and 25fps should not be noticeable; just insert the footage onto a timeline and Premiere will automatically make the required adjustments. Choose the frame rate that makes most sense for your audience, or whichever rate comprises the majority of your footage. Worst case, make two reels — one 24fps and one 25fps.