I've used a chest holster & shoulder holster while wearing my pack with good results. The shoulder holster had to be adjusted right in order to stay out of the way of the pack straps, and it was more comfortable than the chest holster. I found, though, that a chest holster distributes weight better with large guns; a 6" 629 dangling on one side ended up crimping my posture after a few days.

The chest holster I used was a borrowed one from my dad, and I don't think it's made any more. I'd start by looking at the major manufacturers (Desantis, Kramer, etc.). I hear Simply Rugged makes a good one in leather... I bought a belt from them once and think their prices are reasonable.

For concealed carry with a small gun, I clip one of these onto my waist belt: http://www.kgproducts.com/HANDIBAGS.html It works, but I wouldn't try it if your waist belt is really thick. It's big enough for maybe a 3" small framed .357. It's easy to transfer it between the pack, my waist, and a car safety belt.

As far as belt/waist strap placement goes, my strap covers my belt. Both are held up by the hip bones: my belt sits just below the tops of the bones while my waist strap keeps the tops of the bones centered in the middle of their fabric, if that makes sense. I'd think that if the waist strap were high enough to not touch the belt, then the bottom of the strap would be digging into the hips. I adjust mine so it basically cups my hip bones. I'd recommend getting the pack fitted at a good sporting goods store (REI, Cabela's), get the opinions of several different staff, and stick with the method that's most comfortable.

On a side note, when hiking I replace my pants belt with a length of spectra-cored string. It's lighter. Obviously it won't work with a hip holster.