with bigger tank you can over stock (of course not go crazy, use common sense), don't really need to follow the 20g per fish rule.
as long as your filtration can keep up you'll be fine.
I have 7 caribas, 3 wild reds and one piraya in my 180g, slightly over stocked but it looks great, especially during feeding.
besides you should get few more than what you want to keep because you may loose some during the process. if they all live to adult hood and you think the tank is way too overstocked you can always sell them. will take a good few years until they are full grown so you'll have plenty time to enjoy them in your tank before either upgrade again or get rid of some.