I over-stock my tanks according to to the "inch per gallon" rule, but not according to my water parameters, my fish's behaviour, swimming space and all the other variables.
In my 10 gallon tank I've got a male betta, a school of 8 black neon tetras and a pair of cherry barbs (that look abour ready to spawn). There's plenty of room for everyone as tetras aren't terrific swimmers, the barbs stick to the lower level of the tank and the betta likes to sit near the top, front and look out at the humans.
I'd try to make up your schools of tetras a little more - maybe add a couple at a time until it looks about right. They will tend to clump together so don't seem crowded. Incidently, the "inch per gallon" rule refers to length of fish excluding tail so I'm not so sure you have as much fish as you imagine (and also the gallons are US gallons of course, not UK gallons).
If you want some fish that stay near the bottom of the tank and stay small, I'd recommend a pair of cherry barbs. You can add an apple snail as a waste-disposal fish. I wouldn't personally put a dwarf plec in such a small tank and ottos and corys are schooling fish.