Question about planted aquarium and fishless cycling

Common Plecostomus? It will be too big even for your tank. They can reach up to 18 inches in length when they are full grown. Tank size will not limit their growth, and they will outgrow your tank. If you can't get a bigger tank by then, your tank will be overstocked from just this one Pleco.

Guppies will keep multiplying quickly if you do not have other fish to keep their population down. Live bearer will always have this problem. By the way, usually it is said to need 2~3 female guppies for every 1 male since the male will keep chasing the females and stress them out.

Have you checked your water temperature?
By the way, do you have sufficient water surface movement? Either from the filter or from the air pump. When the water temperature is too high, there is not enough oxygen in the water. Surface water movement is required to promote gas exchange between the water and the atmosphere in order to give the fish enough oxygen to breath with.