If you only heed one piece of advice on this forum let it be this....
What you may WANT and what you may GET may be two entirely different things.
Without going into specifics, I will give you a piece of advice that will keep you and your fish happy.
RESEARCH!!!!!!!!!! For about 2 weeks following your new tank acquisition you will be cycling this new tank. If you are unfamiliar with the term "cycling"
do a web search and learn as much as you can.
During that 2 weeks of "down time" use it to research different species of fish. First and foremost find out what kind of water they need (Ph, Gh, temp). Then compare that to what kind of water you have available for them. If you have 7.2 Ph tap water and want to keep 8.0 Ph fish, you will need to do a lot of extra things to your tap water when doing routine maintenance. If you have extremely hard water (Gh and Kh) from your tap, you will again have to make adjustments...not all of which are cheap or easy to do. Again, research is the key here.
Second you will need to find out what the temperment of each fish is....if fish A will get along with fish B.
NExt, find out how big they get when they are full grown. DO NOT fall victim to the belief that a fish will only grow to the size of their tank. Women in China used to wrap their feet in clothes to keep them small and "attractive" but that doesnt necesarily mean its a healthy thing to do. Ultimately, you want to keep fish that, when grown to maximum potential, will still feel at home in your tank.
With those three KEYS in mind, narrow your selection and fish #'s down to your specific tank. If its a 20g or if its a 200g it makes no difference. Just because I have a 200g tank does NOT mean I can house softwater Discus and hardwater African Cichlids. At the same time, it also doesn't necessarily mean I can house 100 neon tetras, 20 guaramis, 30 cory cats, 6 plecos and 10 angelfish.
THe size of the tank, the water requirements of the fish you want, and the compatability of each fish are going to be your limiting factors.
Even with a tank as small as 1 or 2 gallons you can still make an enjoyable and rewarding aquascape that everyone will enjoy.
To make it short, while you are waiting for your tank to cycle, use the time to research and make an informed decision about your future fish. The time spent properly selecting fish will earn huge dividends when it comes to their keeping, maintenance and overall wellbeing.
TO put it in laymans terms, you wouldn't just start throwing raw materials together in hopes of ending up with a livable house to move your family into. To complete a livable project you would need to have a plan from the start....a plan in which every step took you further along to the ultimate goal.
Keep this in mind as you charter on in your fishkeeping hobby and you AND your fish will thank me later.