Can I take power from the hot line on another switch in a multi-gang box?
Yes, as long as you return power on the same neutral that's partnered with the "hot".  (Why is that? Because if hot wires are protected by breakers, but neutrals are not - so if a neutral carries more than its load, that will not be detected.) 

If you are converting to smart-switches, you should be able to convert the hot+2 messengers into hot, neutral, signal for the smart switches.