I'd stay away from this forum for a question like that and call up the manufacturers and discuss it with each of them. I would suspect that a few have some degree of customization available, like Bike Friday for example. True folders (like the Tikit as opposed to a Pocket Rocket) are more difficult to customize. I'd be hesitant to take anyone's word but the designer's on what it will hold, but any engineered device has a factor of safety. A bike is subject to a fair amount of fatigue, so the effects of overburden wouldn't likely be apparent immediately if you were over-stressing the frame or connections. Fatigue failures are usually rather sudden and therefore potentially dangerous. In any case, at your size you'd definitely want to go with a low cost/high fatigue resistant material like steel. As for the height... you'll just have to try them out, again with guidance from the manufacturer on customization.