Fish in a 20 Gallon?

www.msjinkzd.com has a whole host of fish that would work.

It's a great hobby, but like anything else it takes time to learn the ins and outs. So ask away! Further, you will find people with differing opinions on what is ok, you need to decide for yourself what you think will be right. I do have to ask, if you understand the term "cycled". This is not merely running the filter for a day without fish. If you do not, I would direct you to the link under my name to understand the basics. Testing your water is very important to your tank!

When you say pleco I assume you mean something like a bristlenose or other fancy variety that stays small if you are truly interested in plecos. The common pleco will quickly outgrow that tank and is really a waste factory. If you are looking for something for algae controll only, look into some ottocinclus, which can be a fun group of fish to have, and are much smaller. Otherwise, monitoring nitrate levels and having a good waterchange practice alone can usually keep algae in check. I have never been a fan of gouramis (just personal preference) so i can't really chime in there. I am not against keeping some of the smaller South american cichlids in a mixed tank like this, but others will disagree. As long as there are safe places for everyone to avoid agression. Blue rams and apistos can rival many salt water fish. I currently have a blue ram with a few neons and some ottos in a 10 gallon, which I don't really think is too much. However neons(such as the black ones you have) do better in a larger tank with a bigger school. One thing to watch out for, is when starting out, you will want a lot of activity. Over time you will realize it can be much more fun to have a few shy fish that require patience of watching to really appreciate their behavior.

I would never advocate getting a baby fish for a too small tank with the intention to set up a proper size tank for it later. Plans can change and the fish can be left to suffer for it. If you want a rainbow shark, wait untill you get a bigger tank. Average size is 6" and while it will "fit" in your tank, it will be more like a prison cell than a home. The best advice I can give is gather as much advice as you can, make some preliminary choices, then do some research of your own before you buy anything.

I have heard of people keeping DPs in a community but i have never had luck with it. they are sneaky and territorial and will take chunks out of fins. They also eat small snails and things like shrimp, so I wouldn't try it with the ghost shrimp. Most places recomend keeping species only, with a group big enough to spread aggression. My best success however was a single puffer in a 10 gallon tank and nothing else, save for Malasian Trumpet Snails. Normally considered an unwanted pest, the snails will breed like crazy if they have a food source, but with very clean water they can be held in check. The puffer will eat these little guys and it's fun to watch them hunt. I have no experience with the rainbows, so hopefully someone can help out there. Good to see someone willing to do the checking before they learn at the cost of fish lives

I wouldn't do dps in a community. With the stock you have two more corys and a dwarf pleco would work just time. Also, you could add a few more tetras.

I'd avoid adding any fish that gets larger than a couple inches. I just do not like big fish in small tanks. Rainbows get too large unless you get the really small (and still pretty) ones like Furcata or Paskai.
Most any small tetra or rasbora would be fine in a group of 5 or more.
One or more Honey Gouramis are always a nice addition to a smallish tank.