I would not as goldfish do not get lonely-at least that is my understanding although they appear to interact with their caregiver. They actually enjoy their solitude. In a 29 gallon tank, your fish will be very happy swimming on his own and since he is somewhat of a scavenger, he will enjoy cleaning his own substrate/bare bottom tank!
Goldfish are actually extremely social, just with their own kind. Sadly your tank isn't big enough for tank mates. I agree with the above, a pleco would not be temperature compatible with a goldfish.
If the OP is referring to a bristlenose, they are small enough to fit in a 29. Just not compatible with goldfish.