Chis,
Here are a hings to help you out, but I would suggest you spend some time with the AE manual. It will clear up alot of the issues you are having with applying effects, using nulls and even some basics on expressions.
You only need one null. It will control all instances of Glow that have the expression applied. The breakdown is any number of Glow effects, with the expression applied, will be controled by a single slider on the Glow Null.
With the Null selected in the timeline, right click on it and go Effect>Expression Controls>Slider Control. The effect will show up in the Effects Controls window. With it selected, hit Enter and rename it “Glow Slider” Naming is important because the expression refers to this name.
If you follow the directions I gave you, and don’t assume anything else, you should be fine. If there is something in the directions you don’t understand, I would encourage you to find the answer in the manual. It will be well worth the time so you understand the process and can then apply it to later projects.
Good luck!
Mark