60fps and 30fps clips exported into one file?
Export to 30p. 29.97 to be exact if there is the option. Exporting at 60fps is only going to bloat the file size (twice as many frames) per second-- if it's 60p and not 60i of data to encode. 

Secondly, viewers are akin to watching media playback in or around the 24-30fps range. It's what our eyes are used to, so your viewers wont be distracted as to why the 60 fps has less jutter than your 30 fps material. 

Lastly, exporting at 30 fps will convert your 60 fps content to 30 fps by dropping out every other frame. Doing the opposite would mean doubling each 30fps content... and it will not look quite the same. 

There are filmmakers that have rallied for moving towards a higher frame rate, lower resolution per frame exhibition format. The Hobbit was one of the most recent films (only one I can think of) that was actually screened in what they coined as HFS (High Frames per Second). I saw it in theaters in this format, and to be honest, it didn't feel like film because of it. 

Go 30p. 29.97 Progressive.