First things first
Welcome!
You definately add only one fish at a time. But if you plan to add 2 damsels in a 100g tank I guess it would be ok...
Provide some info of your tank, size, weeks in cycle, water parameters, filtration, etc... What is your experience with swt?
Go out there and buy a book that has fish species, compatibility tank size for each specimen, feeding needs etc. If you do not plan to buy a book do a good search on the web, you will find something.
are you planning to qt the fish?
If you are not experinced, and your tank is small, and you just finished the cycle go with more easy to keep-care fish. In my opinion the Hippo is not one of them.
I am sure that there are several people on the board that will help you out...
There are some things YOU must understand, like why you do not add many fish at a time. Buy the book, do not take our suggestions advice for granted if you do not understand what goes on in the tank.... I use to listen to my lfs guy 10 years ago, and I ended up with a screwed up tank, a bank account empty, many hours of stress, and dead fish.
I hope I did not freak you out...