You could keep them in a 75G until they outgrow it. You should upgrade to something wider eventually.... Definitely not an alligator gar as previously mentioned.
nothing for more than 2 years. you can get away with a shortnose or fl/sp gar for that long tops.
2'x5' can get you set for a long time.
for cubans and trops you'll have about a year and a half tops before it starts to get dangerously small.
5'x3' is a comfortable size for these
no way in hell you're keeping a gator or longnose in there longer than a year
ponds are usually the option for these guys.
the growth rate of every individual fish is different and affected by many and sometimes unpredictable aspects of an individual's husbantry/ individual differences amongst fish. so if you feel that your is needing a larger tank, by all means go for it. stunting a fish to keep it in a smaller tank should never be done....