Hi, Cate! Welcome to TFK, and congrats on the new tank!!! Setting up an aquarium is always very exciting - no matter how many times you've done it!!! Glad to see you asking questions before starting - getting a slow start on the right foot is going to go a long way in getting you a healthy and thriving setup that both you and your fish will enjoy for a long time to come!
You're correct in that this tank is too small for a red-tailed shark. . . but as to putting a number on how many fish, or how many different types of fish, that really depends entirely on the fish you end up with!
The size of your tank would be okay for beauties like the Licorice Goruami, but these fish are very sensitive and shy - best kept on their own as specemin fish in a tank designed specifically for their needs. They require SUUUPER soft water, with a very low Ph, in order to thrive, and are probably best in the care of an experienced fishkeeper.
Other fish, like Mollies and Guppies are wonderful fish for a beginner, active and colorful - and tough enough to handle a few new fish-keeper mistakes - but they prefer their water to be on the harder side. So these two would definitely not be compatible with the gourami.
Not all water is created equal, and it can be challenging to shift the water parameters from your tap to suit your animals - especially if you're just starting out. So your best bet would be to start by figuring out what kind of water you have coming out of your faucet. You'll want to know not only the Ph, but the Gh and the Kh as well. On this side of the pond we can sometimes get this information from our water supply company, and if not, there are test kits available so you can monitor this yourself - I'm not familiar with what brands you have over there, but this is something you'll want to look into.
While you're looking into water tests, you'll want to pick up a liquid test kit to monitor the levels of ammonia, nitrite, and nitrate in the tank once you are ready to begin preparing the tank for life. If you aren't already familiar with the Nitrogen Cycle that takes place in the aquarium, now is the time to study up on it - it means life or death to your soon-to-be new fishies! Here is a link that may help to get you started: Bacteria in the Freshwater Aquarium
That's a lot of information to begin with, so I'll stop there before it seems too overwhelming! Starting a new tank isn't *quite* as simple as most people think at first, but it is so worth it in the end!!! I hope some of this helps you out, and I'm sure our other members will be around shortly to welcome you, and answer some of the many other questions you may have! Once again, WELCOME to TFK, and congrats on the new tank! I can't wait to see what you make of it!