what is better for young fish

I'd say just put it in the big tank. Of course its fine to put it in the smaller tank at first, but if you put it in the nig tank right off the bat, you know its growth isn't getting stunted, and you don't have to worry about getting a new tank for it later.

As long as the young fish is not small enough to be considered food by the other tankmates, it's ok.

However, be careful that the new added fish gets its share of food.

It is best to put new fish in a tank alone to make sure they are free of disease. A few weeks in a smaller tank with proper water changes will do them good and give them a chance to adjust.

id just get the biggest tank you can get, saves you time and trouble moving into bigger tanks time after time and it will all pay off in the end.

i say start with the large tank. SOMEDAY, in a few years probably, i want to get probably a 100 gallon tank and keep one or two oscars in it, maybe a couple other fish if i can. rather than starting off with a 50 or something, i'd rather just save up, jump for the 100 right from the start. i think my plan will end up with...

a 200 gallon with some bala sharks for my husband
a 100 gallon with a couple oscars
a 20 gallon with a pair of red honey gouramis and tetras
a 10 gallon with a divider and 2 bettas