[*]Can I sumberge my PH probe?
I believe so... this is what allows it to measure the pH of your tank water.
[*]I have a 29 Gallon tank. How much CO2 should I be injecting?
Depends... enough to reach a level of 15-30 ppm, the higher end is for higher tech tanks.
[*]Is PH all I should be measuring?
To determine CO2? No, you should also measure kH.
[*]How can I figure out the how much CO2 is disolved?
By measuring pH and kH and looking it up in a chart. Many of us also use a drop checker which gives you visual clues.
[*]Is it okay to put my PH probe near my diffuser, filter, heater, etc.
Sure, is okay.