I actually run overstocked in a 2ft by 2ft tall tank. And i can honestly say its fine. Only issue is, if you are fussy about water clarity, you are gonna need to do more maintenance (water changes, filter cleaning) and also at a more regular interval. Fish-wise, from my experience, even if some fish can grow till bigger than your tank, usually 95% of them will stop growing depending on your tank dimensions. Only the alpha fish, or rather the fish that claims the "boss" position in your tank will grow, the rest will still eat but stop growing depending on their size vs your tank size. Just my 2 cents.