So I'm trying to build this bike on the cheap to get to/from work. So far I have a bunch of stuff from my parts bin to build this however came across a $5.99 Shimano Mega derailleur on Nashbar. Anyone have any thoughts on this? It says its for 6/7 speeds. Can I assume that like many of their other derailleurs, I can use 8/9 speed cassette on this with no problems? I'm going to be using an SRAM (shimano compatible) 9speed shifter.