Loaches and plecos and cories, oh my...
OK, there are really two types of cories (on a marco scale), regular and small. Regular cories get about 3" long, prefer to be in a shoal of 4 or more, and stick pretty cloase to the bottom. These are your bronze, peppered, albino, emerald, panda, skunk, leopard, and so on and so on. Then there are the small cories, Pygmy and Dwarf (C. pygmaeus and C. hasbrous respectively). They get about 1" long, prefer a school of 10 or more, and come off the tank floor a bit more often.
Plecos: Couple of types of plecos: too big, ornamental, algae eaters, twig, and Ottos. Eddie mentioned that Common get too big. So do sailfins (gibbiceps - gibbies they're called sometimes). These guys will eat algae off the glass, but will rapidly get a foot long or more, and start trashing your tank just in getting around. They also poop. A lot. Eddie is into the ornamentals. They are beautiful to look at, sometimes hard to keep, and expensive. Some of them get big, some not so much. They tend not to be particular good at cleaning the tank. They also poop a lot. I've got a Clown Pleco
as my ornamental. The Algae Eaters: Bristlenose, Bulldog (or Rubbernose or Rubberlip), and Pitbull. These guys are pure hell on algae and they stay relatively small. In a 29, one Bristlenose or bulldog, or two pitbulls will keep your glass clean as a whistle (most of the time.) Twig: Farlowellas - Beautiful, stately fish, like a cross betweem a sturgeon and a buggy whip. Some will get as long as your arm, and they're persnickety about water conditions. Not for a 29. Finally Ottos: Otocinclus. Tiny plecos, about 1 1/2". They sometimes die for no apparent reason. Like to be in a school of 5 or so. Put them in a tank that is starting to get algae, and they will stop it in their tracks.
Loaches - Loaches are related to barbs. They clean the bottom of the tank like cories. They also, most of them, eat snails. Some of them, the ones that the fish store will try to sell you, either get far too big (clown loaches) or get really nasty (skunk loaches). Yoyo , Angelicus, Striped, Kuhli, and Dwarf Chain loaches are all good in a 29 gallon. Again, they like to be in groups of at least 3, preferably 5 to 7 and in odd numbers. Check Loaches.com for more than you ever wanted to know about them.