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Years ago Shimano made it known that their rear ders were backward compatible but not forwards so. Basically a der with greater cog count intention could sub for one designed for fewer cogs, but not the other way. This survived till10 speed stuff came out as the cable pull specs were the same from 7-9 cogs. I suggest you use the 9 speed Altus.
BTW I suspect the 7 speed freehub body of the STX rear hub could be too short to fit all 9 cogs. Shimano changed to a FH body with longer splines around their 8/9 cog systems. Andy
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AndrewRStewart