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Up to 15 guppies or mollies you could do 12 ifyou only wanted a species only tank.
Ifyou want both In a tank together I would do 6 mollies and 8 guppies cause both are live bearers and the Amount of fry will be huge and mollies are very messy So they better have good filtration
I tend to under stock unless I know what type of filtration the tank has. I would go to aqadvice and check the percentage of filtration with the amount you want and it will say if it will work
I am not sure how many but I believe neon tetras are very small an have a relatively small bioload so they may be a good choice if he wants to have a nice school.