The usual reccomended guideline is aim for 1 inch of adult length fish per US gallon of water. Your tank is 15 US gallons, probably 14 by the time you add gravel and ornaments. That would suggest 14 inches of fish when they are full grown.
The best thing to do is look up the fish you are interested in and see their aquarium needs and adult length and base your stocking on that.
As a smaller tank you want to avoid any fish that get bigger than about 2 inches or need a lot of swimming room.
Are there any specific fish that you would like to keep ?
It's good to see that you've fouind the fishless cycling information, it's a great way to get the tank off to a good start.