25 frames per seconds and 30 frames per seconds footage in one seq
The surest way to make this work is to convert one set of material so the two have matching frame rates. No frame rate conversion is perfect, but moving from 25 to 30 is more likely to look better because no frames need to be dropped. Instead you'll get interpolated or repeated frames. Be sure that your "30 fps" is really 30.00 and not 29.97 before you start. Premiere should tell you, but you can also use MediaInfo or other standalone tool

If Premiere can do rate conversion use it; otherwise try ffmpeg or similar.