Yup, you are seriously overstocked. I see nothing but issues with that many fish in such a small tank.
Can you answer the other questions please? If you dont know, say i don't know.
You need to be removing probably 2/3 to 3/4 of the fish just to manage the bioload.
Lets start with the angel fish. 4 angel fish in 90 litres is asking for trouble. When 2 pair up, they will just fight the others. There just isnt sufficient room for 4 angelfish. Ive tried to keep 4 in 200 litres, and it was fine for about a year, then i had to go to 1 pair or they would have eventually killed each other. You might get away with keeping a pair in that size tank once 2 have paired up, but really even 2 would benefit from a bigger tank. They also might predate on the neons and guppies.
Tiger barbs need a little more space than 90 litres, 120 litres would be better. Keeping 12 on their own in 120 litres might work, but not with the other fish. They are also too aggressive to be kept with peaceful neon tetras.
Do you know what kind of pleco it is? Some plecos get big, a common pleco can get 24" and need a 400 litre tank. Some stay small, like bristlenose plecos.
Honestly you need a complete rethink. Unless you are planning on getting a 400 litre tank you need to lose 2 of the angelfish (really all 4), lose all the tiger barbs, maybe the plecos depending on the actual species, and focus on keeping nice groups of the tetras and guppies. If you are going to get a much bigger tank, then you still need to think more carefully about compatibility between different species.