A lot of experience here....
As far as the material, I agree that pure titanium is BEST! Acrylic or 316L surgical steel is good too, but they're not quite as hypo-allergenic as titanium. And watch out for "titanium" jewelry, that doesn't specify "PURE 100%". Most of it is titanium electroplated, which is useless. Part of the reason why titanium is a must, for big gauges, is that in addition to being biologically hypoallergenic, it is light as a feather, and the weight of the plugs (or rings) in other materials will exacerbate or even cause problems.
It's normal when stretching; particularly from the 6g-00g range, to have soreness and rupturing with a new healing process setting in. The skin and tissue actually have to rip and heal to accommodate the bigger size. Once you get past 00g and move up to 24mm where mine are, it won't do that so much. Mine will shrink a little overnight if they're empty, but as soon as I take a hot shower, I have no problem reinserting the plugs.
I know we're talking gauged ears here, but I should mention also that titanium is A MUST for nipple piercings. Many people will tell you that acrylic, niobium, or 316L is fine, but trust me; you want titanium. It's costly; 2 or 3 times the cost of other materials, but just consider it a necessity if you're going to pierce anything.
Charles