If you started with brick coco, or bagged that didn't specifically say it was pre-charged with calcium, you'll need some cal/mag. Get a bottle of Botanicare it's cheap and will last a yr in a 4x4 tent. (you'll find 4 plants will be max for the space)
Yes you can wait several days between waterings when growing hempy's. Frequent waterings will keep the medium above the hole at a great environment but you don't push out much of the rez and will get concentrated, especially if you are feeding at a hi level. Coco has great wicking properties and will keep the medium above the hole moist and working while emptying the rez. I'd recommend tipping the buckets and draining as much of the rez as you can without spilling medium. Wait a day then water normal levels of nutes. (1.5 gals.... you'll have at least a qt of overflow) then wait to the next watering until you can lift the bucket and feel it's dry.
Yes I agree, top at least once, it will make a huge difference to height, and being you're in 5 gal buckets I'd top twice, since it will take a while to most out of the big bucket.
You started in a very large hempy bucket for a first grow so I wouldn't wait to get full potential. Just top as KK mentioned grow to16" to 18" above bucket, then flip. They'll finish in the 4'+ tall area, with your light and rm the finish should make you happy.
Best, Watts