tank sounds like an interesting one lol. I dont see to many lionfish with corals. Certainly a tank id love to see pics of. Maroon clowns and lions actually make very good tank mates, as each grow at a rather decent rate. Now with that said, id still follow your original idea and grow the clowns as mush as possible first and then add to the tank, preferebly before the lionfish.
Tangs: now i owned one in a 3 foot long tank for about 7 months and yes he was active but he also grew and out grew the dimension of the tank, not the volume itself. A 75 is usually the minimum recommended lenght for some of the smaller tang species. Now if you had intent on keeping unicorn tang, achilles tang, etc then you may want a larger tank, just for that added space for them. Just my thoughts on the subject. The tank in my experience will grow as long as provided a healthy diet of greens and some meat and a decent amount of space.