I'd be nervous adding anything to a 29 with a pair of convicts (it has the same footprint as my 20 long). I'd try it with a single con, but I'd watch the tank closely for a few days since cons can be nasty (that's part of their charm).
As for plecos, go with a rubberlip or a bristlenose/bushnose pleco. Both stay small enough, but the bristlenose is more common. Also, both are often used in rift lake set-ups because they do well in high pH. Just give them extra time in the bag before release into the tank, slowly adding small amounts of tank water to the bag (I do this over 2 hours) since I think pH shock can be a big deal for any fish.
Just my .02.
Eric