I'll try to explain,
I have 4 cams in a nested sequence and am now editing that sequence in the multicam monitor.
2 of the cams are 50fps and 2 are 25.
After cutting it up, there are parts of the 50fps footage that I want to apply slow motion too.
But when using the rate stretch tool it becomes apparent that it has now become 25fps and the slow motion is rubbish.
To maintain the use of the 50fps, do I have to make the initial sequence at the highest framerate of any one cam in the sequence? So, eg. 6 cams- 5 at 25p, 1 at 50p therefore = 50p sequence?
Would this affect preview of the 25p cams?
Is there another way?

Perhaps it would help to set the sequence for the majority of the footage and then use "interpret footage" feature to adjust the one odd clip's framerate to match the others.

Changing framerate in the interpret footage dialog will only result in a different playback speed. Not sure that's what you're after.

I'd make a small test with 50p sequence, and see how 25p footage looks there, but I think it will be a valid - and most likely the only - option if you later want to retime.