Hey bro ur perfectly fine! Good question! U are going to want to try to veg at a foot tops. Keep in mind u can top ur plants soon as they get there sixth true set of leaves and keep growing them out ill u hit that foot! Thereby allowing ur plants to grow more stems and colas for more buds!
I can tell u that in experience, u can easily grow 6 plants in ur 18 gallon tub vegged at 1 foot tall. Yes u will end up at the time of harvest with almost a completely full tub of roots, completely entangled mess, but As long as u change ur water as suggested ( every 10 days- 2 weeks ) and feed as directed on ur nut bottles u will be fine!