Reduce the hours your light is on, use a mag float, or a standard scrubber to get that algae off. Algae probs are typically due to an imbalance in the tank. Yours is very overstocked which may be contributing to your issues. Adding another fish would only make matters worse.
The barbs an ram need a tank about twice the size of yours, and the pleco may need even larger depending on the species. I suggest you rehome these fish or get another larger tank for them. You can take a pic of the plec and post it so we can help you ID which species it is and what tank size it needs. As you've already discovered, not all plecos eat algae, some are carnivores, and some are wood eaters, so you really need to know which species you have so you don't starve it to death. Plecos create huge amounts of waste so it really has the potential to wreak havoc on your tank.
The neons (you should keep a school of atleast 6) and a few guppies would make for a nicely stocked 37 liter. With just them in the tank, you could add some snails or shrimp to help keep it algae free.