I don't know if this is the right place for my question, but I give it a shot.

I acquire in nano I-Frame only @ 180 mbps or higher. Usually in 1080p25. For my project I do however also have quite some footage at 720p50/60, converted to 25p (thus for slomo effect). So a significant part of my project is 720p. If intended for broadcasting, should my workflow further be based on a 1080p timeline (and uprez the the 720) or on a 720p timeline (and downrez the 1080)? I work in FCP by the way. Any recommendations?

I believe most of the broadcast is 720p anyway. Due to the image quality/bandwith ratio this format was embraced by a lot of broadcasters for HD.
I suggest stay at 720 and downconvert 1080.
Don't forget that the EXcams use a down/up trick for the sensor to output 720. As it is 1920 native these images look better than 720, specially if you use snapshots.