What tips are you struggling to get on? The stock ones all fit fine - onto the earpiece stems that is.
I was using the green cored hybrids, but I think the red or orange will be better for me - I don't have any to hand right now but will get set later and test. I think the re-400s sound their best when you have as deep insertion as possible, so the housing of the earpieces nozzles your tragos.
the stock smalls. couldn't get the stems on. did eventually get them on and i guess my canals are too big/shallow for me to get any sort of seal. but i put on some green core hybrids. not really sure if this is a big improvement over the light blue cores i usually use. i'm getting less of a seal and i'm not sure what differences i hear overall so far.