Depending on heat and humidity in your area, you might need to drill some holes in the sides too. If lower root zones are not allowed to dry between waterings, you risk root rot.

I fill my 5 gallon buckets to within 2" of the top to allow for watering, but as previously stated, you'll want good air circulation at the soil surface. Topping, fimming or tying-down the tops can control height.

Are you starting them in the buckets, or working your way up to them? Immature plants in a 5 gal. bucket is a waste of nutrients and is more difficult to properly water.

With only 5-6 feet of headroom, you'll lose about 12" for the light fixture, the distance from the light to the tops about 8-12" (depending on temperatures) and the height of the bucket (about 12-16", I believe) leaves you enough room for about a 2 or 3 foot plant. With a sativa dominate strain this can be problematic.