So after having this huge pedalboard for a while, I've decided I'm going to switch over to a rack setup. This is what I'm going to put into it;

A Furman Power Conditioner
A tuner (no idea what kind yet)
A Digitech GSP-21 Legend
A Digitech IPS33

And at some point I'm going to put a wireless unit in it, but that won't be for awhile. All of that is going into the effects return of my Peavey Windsor

I'm a little confused as to how I'm going to setup everything with MIDI. If I'm not mistaken, MIDI is when you can change settings on multiple rack units at once, right? Like clicking one button on my foot controller could change the channel on the MP-1, turn on/off delay and reverb on the GSP and change the settings on the IPS33, right?

If that's so, could I use the foot controller from the GSP-21 to do all that with? Or would I need to buy a special foot controller for that?
I've never used the GSP-21 but there is an easy way to find out if it will work as a controller. Look in the manual towards the back. There will be a chart called MIDI Implementation or something similar. That table will tell you if it will transmit program change and controller information. If it does then you will be able to use it as a controller. If it only receives that info then you can't.
Gilchrist custom
Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band
I found the chart, and where it says "Control Change" it has an X for transmitted and an O for recognized, so I'm guessing that means that it can't?