please put in more then 2 shrimp, they are a grouping animal and they are a lot more active when in groups of 5 or more, this wouldnt effect the tank at all other then the more active and more brave shrimp, my shrimp actualy come to the front of the tnak when i come in to say hi :)
watch what kind of pleco you get the common plec start small but can end up around 1ft dif kinds stay around 4 o 5" look into bristle nose and i have a pleco and six swordtails fishtank life is good and they come in alot of variations
I wouldn't put more than a single dwarf gourami in a 20g tank (unless you can get your hands on a pair, which is hard to come by in many places). They're a territorial species that doesn't get along with multiple fish of its own kind.
Definitely stay away from multiple dwarf gouramis unless you are getting a male and a female, a male and 2 females, a male and 3 females, etc. It doesn't matter how big your tank is, dwarf gouramis (espescially males) hang out at the surface so even though they have the whole tank to 'claim', both males will end up sticking to the surface and 'getting in each other faces'. I have kept DGs in a 20g before, trust me, putting multiple males in is just a bad idea.
I would not keep more than a total of 3 dwarf gouramis in a 20g. I can see you are following the 'inch per gallon rule'... this works in SOME situations but not all. Essentially how many fish you can put in a tank depends on how much poop the fish will produce and whether that poop will accumulate to dangerous concentrations when dissolved.
For instance, a dwarf puffer (1" long) needs 5g to itself because of all the waste it produces wheras those ghost shrimp could get along just fine with only 1/2g each of tank space.
It would be a nice presentation to have 1 male betta or 2 female bettas
(heavily planted tank with lots of hiding places if you want to try a breeding pair)
Ghost shrimp (may get eaten by the betta... if you'd rather have the shrimp, replace the bettas with something. maybe 1 dwarf gourami.)
and a large school of tetras... Maybe 6 cardinal tetra and 8 Ember tetras.
I think that would fit if you had a ton of plants. :p