Hi Denise, and welcome to Fish Lore

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Yes, it would be best if you took the 2 plecos back because they will get too large for a 29 gallon tank. I think a fish like pleco needs AT LEAST a 75 gallon tank (I'd personally get even larger tank for it).
Now, don't get any more fish until your tank is cycled. Four tetras in a 29 gallon tank is far from overstocking, BUT any fish in a new tank may have to suffer because of ammonia and
nitrite. A tank usually cycles around 4 - 6 weeks, but if you add fish to a new tank it may cycle longer with fish. It's always best to cycle the tank first, and then add fish. Your tank will first have ammonia, then nitrite, and lastly,
nitrate. It will be cycled when ammonia and nitrite = 0, and when nitrate is present.
Because you already have fish in your tank, once you have ANY ammonia and nitrite in your water (however small these amounts may be), you'll need to be performing 50% daily water changes ... until ammonia and nitrite = 0. This is the only way to protect your fish from getting sick from ammonia and nitrite. Even smallest amounts of these 2 compounds can make your fish sick or even kill them. And even with daily 50% water changes, I can't guarantee that they'll make it. However, they will have much better chances of making it with the water changes than without them.
Once your tank is cycled, then you'll need to perform weekly 15 - 30% water changes, as a regular tank maintenance, so that nitrate is regularly removed from the water. Nitrate isn't as dangerous as ammonia and nitrite, but it can also become toxic when in large concentration. Strive to keep nitrate below 20, and 0 at best. The maximum is 40
PPM for nitrate, but I wouldn't let the water get that dirty.
If you want a good algae eater for your 29 gallon tank, get a few Otos, or 1 Siamese Flying Fox, or 1 Bristlenose Catfish. If you get the Siamese Flying Fox, MAKE SURE that its scientific name is
Crossocheilus Siamensis because oftentimes the store will sell a loach that looks like it but is not it.
I guess that's it for now. As I've said, monitor your water for ammonia and nitrite now. Once they appear, perform daily 50% water changes until they = 0. Give the 2 plecos back to the store. And lastly, when you perform water changes, vacuum the gravel with each
water change with a siphon tube (available at every fish store). Good luck

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