*nods and agrees with Scott on the red tail shark*
Not great community fish. As Scott said, REALLY aggressive buggers; you can only keep one to a tank as they'll likely kill any of their own species, and they don't play well with others. Not a good choice for peaceful delicate lil guys like neons.
Do be VERY careful about adding more the one dwarf gourami. Also aggressive within their own species (although they do well with gentle fish like neons); they too have been known to kill each other. That being said, I also have had success with keeping more then one dwarf gourami in my 10g tank, but its really an exception to the rule, and I have homes for them already planned should WW3 break out. If you decide to go that route, make sure you have a backup plan should you need to remove them.
Bottom feeders/algae eaters/cats. Lots of choices there. I'd start off with a school of corys; 5-6. Fun to watch, and fairly hardy. I'd wait if you are looking to add an algae eater as well, at least until there is algae for them to eat. You could either go with a bristlenose plec (which tops out at about 6 inches), or a few otocinclus (which remain small; about 1-2 inches) but if you go with otos do not add em till the tank is mature. The bristlenose will eat algae wafers as supplements, but the otos can be fussy and you want to be sure they don't starve.
I've the dwarf gouramis, a small school (5) of neons and 2 otos in my 10g. Really nice mix of fish. I'd suggest something similar in your tank, a single dwarf, a slightly larger school of neons, a few otos and adding the corys too. This way you'll have fish at all levels of the tank, and always something to look at
Do keep in mind my tank is overstocked (but is very well planted and water parameters are monitored regularly, along with weekly water changes) but is half the size of yours. Oh and do keep in mind neons can be tetchy; don't be surprised if there is initially a lot of die off. Do QT all your guys first; better to treat in a QT if you need to then to mess upthe main tank.
Lastly, an emperor400 is a bit much for a 20g. Figure you want around 6-10x tank capacity flowing thru the filter each hour. A 400 would give you 20x the tank amount! Overfiltering is never a bad thing, but that might be overkill; you could easily do a smaller filter safely and still overfilter (and save a few dollars as well).