Hi! I had the same trouble with the grey flexes and the flanges, after some time they hurt. The fit in customs is perfect and the silicone is very comfortable so you can spend hours of listening whithout issues, the isolation is outstanding, the increase in bass impact, clarity, detail,treble and soundstage is remarkable I was thinking about buying some customs or adding 3 drivers to my um3x but I'll wait till the um3x die...I'm very happy with the sound of the 535 now, before I was very disspointed with the bass impact, I didn't expect this change, they seem another set of iems and for good of course
You can use them with other iems with the same nozzle like westones or shure 530 but remeber they made the tips for the 535 so they also made them counting the ergonomics of the body of the 535 so the fit is only perfect with the specific headphones, I've tried them with the um3x and they fit good but I didn't like the soundquality of um3x with them, and sometimes the nozzle went out the tip...more than often I should say, but once you get a good fit they work very good the only thing I didn't like with the um3x as I said was the soundsignature anyway the soundsignature of this headphones is weird. I guess that this kind of tips should deliver the sound of the headphones as they were designed.
Hope I've been helpful.