Welcome. As Joyler has said, a twenty nine is too small for a common. They reach at least eighteen inches, and a one hundred twenty five gallon is considered as the minimum for them.
There are wonderful, smaller plecos such as bristlenoses, what I'm getting in my twenty nine, and rubberlips. Both stay around six inches.
Again, welcome to AA, and enjoy the forum!
PS: I also was going to get a common for my twenty nine until somebody here told me about bristlenose plecos. So I'm not looking down on you, I have been in the exact same situation.