I agree with the others, if you do decide to get the Potter's make sure it's eating well, fat and well-coloured. Also watch its behaviour - if it's hiding, spooky and not picking at the decor, forget it. A healthy Potter's should be bold, bright, inquisitive, fat, constantly looking for food and chubby.
I went through three Potter's angels before finding one that was truly healthy, but I'm glad I persevered. She's a stunning fish with a lovely personality, and easily the most eye-catching thing in the tank. We've had her for months now and she's healthy as a horse - once they're established, these fish seem to do very well. I believe mine has done so well because she was at the LFS for three weeks before I bought her. Potter's don't ship well, and a rest period seems to make all the difference in the world.
As for keeping a Potter's with your rusty, I don't forsee a huge problem. Potter's tend to be mild mannered (compared to halfblacks, lemonpeels etc.) and you added the shy rusty first. If there are plenty of hiding places and you have lots of rockwork for grazing (another must for Potter's angels IMHO) they should coexist without issue. I've got a feisty little coral beauty and he and my Potter's get along just fine, despite the fact that they look quite similar and are nearly exactly the same size.