They could all work well, depending on the size of the tank and the water.
Plecs can grow to 2 foot long, depending on the species. If you have a 60 litre + tank there are several smaller plecs, such as the bristlenose and bulldog plecs, that would be fine.
Tetras can be nippy so I'd pick them or the guppies to save the guppies' fins, but many people keep them together well.
Mollys need hard water, so if your tap water is acidic or soft, I'd avoid them.
If you wanted to keep all those guys together, a well planted 70 litre tank would be a good option. Make sure you get a small plec, and not just one that looks small in the shop! Max size 4 inches.