As far as I know, contacting the ball below waist level is permitted in padel, as are open and closed stances. Having both feet behind the service line is also legal during the service motion. I've used a continental grip and contacted the ball low for a kick serve effectively before.
As long as you don't step over the service line, having both feet behind it is fair play. The key is finding the service stance and contact point that allows you to deliver the ball into the service box cleanly. Experiment to see what works best for your swing mechanics and preferred serve strategy.
