I initially found the design slightly off-putting, but after using it for a while it's gotten more natural. Tip choice is also very important as it both provides seal and placement/compliance of the whole assembly. For example a Comply T-400/T-500 would work very well or a Shure Olive tip transferred to a Comply or similarly sized tube.
I do assume the material could be trimmed relatively easily as long as you felt comfortable doing so and knew where you wanted to trim from. I'm not sure what's available for replacement "gummies" in case you went a little hog wild and completely ruined a pair.
---- just adding some additional comments after more use ----
I do consider the RE252 to be more of a reference product. I would call it technically a better sounding produce from a raw reproduction standpoint. It simply is in my opinion. The transparency is better, you pick up smaller details and nuances better, and the sound stage is better spaced. I do find the RE252 sensitive to the quality of the recordings. I've listened to music that made the RE252 sound like complete crap, and I've listened to music that make the RE252 sound remarkably good. It's not really a genre fit either as that typically is a side effect of a skewed frequency response. It's simply a matter of what shows through.
The CK10 is a a more likable earphone. It's one I can put in and immediately enjoy the music. It is slightly forward and engaging but not too forward. It is fast paced and high detailed but also not harsh or piercing, although there is a response bump that peaks at 12kHz that does make for a slightly hot sound. Still the top end quality is super refined and controlled so what is strong isn't really all that off-putting when it probably would be on a lesser earphone. Besides, a little EQing can go a long way for that kind of stuff, and the CK10 is stellar with any music when balanced out. It has just enough of everything to go "hey this is interesting" and you just can't stop liking it. I'd probably pick the CK10 over the Triple.Fi 10 if I had to choose one to keep between the two. It is that good. About the only desirable thing one would give up from the Triple.Fi 10 is the strong bass line. It's just has a thicker, warm, more robust bass end, but that bass doesn't transition to the midrange very smoothly, and the CK10 is far more seamless throughout.
I would still pick the RE252 over the CK10 though. That's just me. I'm a bigger fan of the little details. I actually took off my RE252 to hear a conversation I was hearing in the background, and then I realized it was background noise in the audio track. I've never done that with any other earphone but my OK1 buds. An earphone being able to fool me into thinking what I'm hearing is real impresses the hell out of me and it seems to be insanely rare to find. The IE8, UM3X, SE530, TF10, ER4S, PFE, Custom 3, etc. could never do that, not once. Unfortunately the CK10 doesn't either. Still, there is a very dark side to the RE252. If you listen to bad recordings, it will sound bad. At least with the CK10, even the less then ideal recordings remain enjoyable. Frankly, the CK10 is always enjoyable to listen to. The hot top end of the CK10 can be slightly spitty with some vocals, but EQing fixes that kind of stuff right up. That's literally the worst thing the CK10 does, and that's not bad at all because the quality of sound on the top end is so good.