my suggestion...if you are just working with 60fps footage, don't conform any of it and keep everything in 60 frames in a 60fps timeline. you can slow down a clip 50% or maybe even a touch more without any weird glitching or lagging. it will still be smooth because it is in a 60 frame timeline. make sure you uncheck the frame blending tho when you slow it down. i believe lots of people on here edit in a 60fps timeline. ive done it when working with all 1280x720 @60fps footage and it works just fine. then when you export, export at a 24 or 30 frame rate and it will look very smooth.
since you have already conformed some of your shots, i would edit in a 30fps timeline instead of a 60 now. you might get some weird frame blend/drops with the two. i would almost recommend conforming everything and just speeding that back up if its not a lot because sometimes that can look a little staccato i think. but in the future, just keep all your 60frame stuff together and slow it down in the timeline. it will look just fine. i only use conforming when im working with 1080p @24 frames and want to slow something down thats shot at 1280x720p 60fps.
hope that helps. maybe do some test sequences with both methods to see what works best for you