RE: How long is too long?
1/4" nylon bolts are fine, and the slight amount of ovality in them greatly reduces the chances of them loosening with vibration. If you crash hard enough to snap the Nylon screws, you're going to have major airframe damage anyway. Stronger steel screws would just ensure that the landing gear struts are even more damaged.
I usually hold mine on with #8 steel screws and tee-nuts, but only for the convenience of the tee-nuts. Nylon-insert locknuts are really approriate for securing the float strut pieces together. I have landed with a float dangling and the plane survived, but it made my knees all wobbly and I would prfer not to repeat that experience.