Reach Comparison
 

Assuming everything else is equal...

If I comfortably ride a bike that has a reach of X with a 130mm stem, would I also fit a bike with a reach of X+30mm with a 100mm stem? Again, assuming everything else was equal, would both bikes feel like they had the same postion/fit?

All else being equal, you should be able to match the fit. Handling may be a little different because you're changing your F/R weight distribution a bit, but a 100mm stem is still within the "normal" range.