You are right, nothing worse than spending money, only to find out that it is not working.
However, IF (!) it is a Shimano system, I am rather sure all horizontal spacing for 7 gear cassettes is the same, even for various brands.
That means that any Shimano 7 speed shifter will provide the necessary indexing.
The tables you refer to is more important to match the derailleurs with different sizes of cassettes and chain rings and different brands.
However, in this case the whole drivetrain remains the same (or so I understand), so the required indexing from derailleurs and shifters is the only one to watch.
Finally, all shops I initially contacted told me I could not upgrade my bike to 9 speed, and some even refused to sell me parts.
Well, I am a bit stubborn, and did it anyway, even using the original rear wheel + hub, just moving the hub on the axle, and re-dishing the wheel.
Moral of the story: ~~~~ 
