Mixing frame rates

I make videos where I overlay generated data (heart rate, speed, distance, etc) as gauges on GoPro videos. I make the generated data on a constant blue background (not green because some of the gauges use green) and use chroma key to show the videos "behind"or "through" the gauges. I've recently started shooting the videos in 4k/60fps but the program that generates the gauges (Garmin VIRB Edit) is having problems with 60fps. Would it degrade the quality if I make the gauges on 4k/30fps? Does it just double each frame in the 4k/30fps if I put it in a 4k/60fps project?

Semi-related: GoPro calls it 60fps but FCPX says it's 59.94fps. Should I make my blue background at 30fps or 29.97fps?

I guess I'm not quite clear on what effect you are going for. Why is your project 60fps?

For sound speed you would have a project that is 29.97 (or 23.98) and then your footage would be the same. If you want it slow motion you would shoot at a faster fps and drop it on your project.

Yes, if you put 30fps footage on a 60fps project it will have to double each frame (via interpolation) and won't look great for any moving images. And yes on the 59.94 or 23.98 in FCPX for standard frame rates.

I'm shooting at 60fps because there are a lot of dynamic elements (water drops, paddles, etc) and I just think they look smoother in 60fps.

Unfortunately the program I use to do the data overlay (Garmin VIRB Edit) will only export at exactly 30fps, which worries me when mixed with GoPro footage at either 29.97 or 59.94.

I understand. It may be that your overlay image doesn't have enough movement to be odd looking on a higher fps project. I'd give it a try. If you notice any ugly motion "smearing" from the interpolation then you can always drop back and use a 29.97 project. Good luck!

such overlay data is (mostly) not animated - so why care for frame-rates???