The rule of thumb(I won't comment on how useless it is) is actually an inch of fish per GALLON, not liter. So, round it off to one inch of fish per 4 liters. Only really works with small tetras.
I think I'll make up a cubic neon rule of thumb. One 1" neon is 1 cubic neon. a 2" fish is worth 2 cubed neons, so 8 cubic neons. So, my 8" severums would be 8^3 cubic neons. So that fish is worth 512 cubic neons...
Hmmm...Maybe I need to rethink this...Ok, we'll make it square neons!
So that way a 2" fish is worth 4 square neons and the severum is worth 64 square neons. So, now that we've got a baseline, and say that a 220L tank is good for 2 severums at medium stocking that's 128 square neons worth of fish per 220L. So, let's say moderate stocking is 1 square neon per 2 liters.
Now, going by that and making some guesstimates at the size of your fish...
3 clown loaches@3" each=27 square neons
10 neons=10 square neons
1 bristlenose@3"=9 square neons
1 borneo sucker@2"=4 square neons
3 cories@ 2"=12 square neons
2 mollies@ 3"=18 square neons
2 baby
[email protected]"=.5 square mollies.
So, that's a total of 80.5 square neons. Your tank, being 180L has a capacity at moderate stocking levels of 90 square neons you've got room for another 9.5 square neons.
Hmmm...Something tells me I may have overthought this a bit. hehe, Generally though, just keep an eye on your nitrates. You're overstocked when your water changes aren't keeping your nitrate levels in a reasonable range. So, you can either do bigger water changes to not be overstocked or remove some fish.