I have a Suntour Blaze NOS RD, it looks like the cage capacity should wrap 28t with a 28t largest rear cog, that I believe will index with your shifters, either six or seven speeds. PM me if this would be useful.
I would guess, however, that your Superbe shifters and XCE RD (even if set up to work with Shimano) should index a Suntour seven speed fine, based on spacing info here...
http://sheldonbrown.com/k7.html .
I can report that Suntour Accushift 7 speed shifters index a Deore LX RD and Shimano hyperglide cassette very well. One of the keys to using the XCE RD I think, based on the spacing, is seven speed--its just about identical between Shimano and Suntour.
Too, your derailleur cable housing must be compressionless. It should make a sufficiently big loop coming off the chain stay so that it enters the RD barrel adjuster in a straight path--as opposed to entering at an angle. If you have bottom bracket braze ons passing the RD cable thru to the chainstay, you also might want to consider a three or four inch section of teflon housing to minimize friction--a great enemy of indexing.
Good luck.