Although people do stack vented and non vented kites, it is normally not suggested by Revolution to do so. The different kites do fly differently in the same winds which can cause the stack to not fly quite as good as a stack made up of all the same style kites. All sails will stretch out over time depending on how they are taken care of and what the wind conditions they are flown in. The only time this would be an issue is if you did fly them in a stack, or side by side....but, they would all fly differently anyways so it would be nearly impossible to tell if the difference in flight was caused by the stacking, or by the normal characteristics of the kites designs, or by the use/wear and tear/condition of the kites as they are flown? Will they stretch out differently, yes....what is the consequences of this? They would eventually fly differently when compared to the other. Is this a big deal??? Not really, they already fly differently from each other new from the factory anyways.
In reality, your kites will slowly change in how they perform over a long period of time, that's just how it works. Things get warn out the more you use them. You will adjust your flying skills/styles to compensate for the changes in your kites performance. Sometimes the performance is better the more the sail stretches (think of it as a pair of shoes getting broke in to match your feet as you wear them). I personally don't think the wear-n-tear on the kites being stacked together is going to be any more or less than flying them solo as long as you pay close attention to the wind conditions you are flying them in.
Hope that helps.