If by “Full HD” you mean 1080p, I would avoid that as your 720p footage up-ressed will look terrible. If your final delivery is PAL, I would edit in 720p then just output the video to PAL standard upon final export. This can be done out of Premiere.
As far as your frame rate goes, I much prefer the look of 24 (or 25) compared to 30, but in this case I would probably go with 30 as that’s less conforming for the 50fps material. If you create a 30fps sequence and just drop the 50fps footage in there, I believe premiere will just drop one out of every few frames in order to make it fit. Then you can conform the 25 footage to 30, which will result in a slight pitch shift in the audio, which can be easily compensated with Premiere’s pitch shifter. I would make sure to keep all of the 25fps audio on it’s own track throughout the sequence, then you can just apply the pitch shifter to the audio track via the audio mixer.
I’m sure you’ve figured this out by this point, but it’s wise to think about your settings before you shoot, so that all your footage has the same look and you have much less technical problems in post.