As long as you use a quality purpose made hose clamp, there is a negligible risk of damage to the hoses. Locking pliers with their serrated teeth tend to damage the hoses.
I've never been been a fan of bleeding through the union, with the manufacturers providing a bleed nipple on the caliper, it smacks of a bodge to me. Just my opinion though.
I wouldn't worry about trying to prevent fluid loss and treat it as an opportunity to change the brake fluid.
Regarding painting calipers, each to their own but make sure the bleed nipple is freed off and perhaps given a dap of copper grease before painting. I say this because I had a nicely painted pair of calipers brought to me with a sheared off bleed nipple in each. Managed to get the remains of each nipple out without damaging the paint!
Arwyn
