Bethy, welcome to the forum! And welcome to the world of h/j's! One of the hardest things for a lot of people who switch is the different leg position, esp heels/ankles. While yes, generally most peopel will say "heels down" I don't think "ankles down" is necessarily wrong. First of all, you'd be amazed by how many people, when they hear "heels down", shove their heel down as hard as they can which pushes their lower leg forward. I generally say "relax your ankle and lengthen your leg". Gets the rider to sink into their heels while keeping their lower leg under them. Sounds like "ankles down" is about the same concept.
As far as stirrups go I wouldn't look for anything gimmicky like wierd shaped pads or anything. Some people like jointed stirrups (like the Herm Sprenger or Metalab) and some people hate them. I think they're fine, but I don't think they will really help your ankles loosen up. The problem is a tight stiff ankle and the best way to fix that is practice practice practice! Instead of looking for different stirrups I'd try this exercise at home... find a step somewhere (preferably one that has a hand rail like a staircase) and stand facing the step. Place your feet so the balls of your feet are on the step and the rest of your foot hanging off. Slowly raise up on to your toes, and then gently sink your weight down into your heel. Slowly up and down several times every night. You should feel the muscles in your calves start to get a work out and the tendons in the back of your heel will start to stretch. Don't get discouraged, it's a very common problem you have! Good luck and have fun!