Pro-tip: When setting up your 75 gal., temporarily take a handful or so of the substrate from the 29 gal. Beneficial bacteria lives on and in your substrate and will jumpstart your cycle in the new tank.
Also, what kind of Cichlids in the 55? I don't think Africans are compatible with plecos because of much different water condition requirements. A lot of South Americans are, but not all. In my tank, for example, Firemouths are usually just fine with plecos...unless you wind up with a mated pair, in which case they become VERY territorial. In my case, they killed a good-sized pleco that was bigger than them.