When I got my first aquarium at Petsmart I was told by the guy who worked in small animals (specially trained in fish dept) that 3 mollies, 2 platys, 3 guppies and a Chinese Algae Eater would fit well in the 10 gallon aquarium I had in my shopping cart.
I must say what I learned from that and the experiences shortly following...
1. NEVER listen to an employee who pauses as you're squatted peering Into rows of fish "ummm I don't know, they should get along fine"
2. 1 male Molly will harass the $#!¥ out of your remaining female Molly once you discover the other female is sick and had to take her out...
3. You must buy several more female mollies for your female Molly to not die from stress from your male chasing and mating with her almost constantly...
4. Two male platys will fight and argue over territory forcing the weaker one to stay behind a plant, only to be chased back if he tries to come out..
5. One guppy will die... Perhaps cause he was the only yellow in the 3 or maybe because his name was Tinkle and he was mortified...
6. The remaining 2 guppies will continually try to mate, and be successful in doing so, with your small female Molly... Swimming sideways and upside down to try to make it work out in their best interest... Forcing their aquamom to look up the possibilities of Muppies or Gollies...
7. A Chinese Algae Eater will KILL MY FISH! (so glad I researched him prior to this happening). I was told he would keep the tank nice and clean... Anyone who knows about these fish know they get up to 12"!
8. You find out what it means to be square, how long it takes for a Molly to drop her entire batch of fry, that it tickles so much getting swarmed and your nibbled upon by 100 Molly fry, and how excited a grown person can get suddenly findin two little microscopic eyes peering out at them from inside a plant..
9. Last but not least... You realize you're obsessed!!!