people say to put them in when they are bigger than the other fishes mouths, but i dont believe that. when i had only molly fry in my 20 gal and i wanted to introduce yellow/ red platy fry that were a tiny bit smaller than the mollies, the mollies would not except them. i saw that it was the color change. when more molly fry was added, they were fine, but the bright red platies were like swimming meals. i think i lost a platy due to the fact, so i waited longer to put them in. and when i did add them i scooped them in the net and put them in the new tank (still in the net) and then i gave the mollies food, so they naturally would go to the top and the new fish could go safely to the bottom in the plants. do you have a pic of how big happy is now? i mean, you could put him in any time if you had many "fluffy" plants (not one with big leaves) for him to hid in. but i would be afraid of bullying still.