Hobbies? Well, kites is the main one of course, but along side is Land Sailing & Buggying. But if the wind toys are not being played with then it is metal working, wood working and web design that takes up most of my time. After that the video games come into play.
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.
I would suggest going to 150 or 200 for the stack, just to be on the safe side. Breaking your 90# set will do two things that you wont like, obviously it will break your 90# lineset, the other is that now you will need to purchase a new 90# set and then also purchase a 150 or 200# set, costing you twice as much as before.
The center stack line use to be a real pain, but Rev has now helped out with that a ton on the new stack line sets. The center line is specially colored so you can easily identify it from the others. What you want to do is to loop it through the center loop on the front kite, passing the loop through and then pulling the rest of the stack line through so it makes a larks head on the front kite, then just your standard larks head knot on the rear kite on the center loop where the knot is. Hopefully this will make sense when you look at it. If not then let me know and I may be able to get some pics. The new stack line kits should have the full instructions in them on how to do this.
Yes, 300# would be the lineset. For stacking more than 3, you may want to go with a heavier bridle as well. Stack lines usually aren't an issue because you are using 7 of them so the 180# stack lines should work no problem.
I have never tried double stacking the leading edges in a stack, just used the larger stiffer rods. I leave all my stacks together though and don't take them apart to fly solo so it isn't an issue.