String size isn't a competition. Find what works best on your bows and that you have confidence in and then go shoot the thing. I like 6 and 8 strand endless loop Ultra-Cam strings so those are what I use. It's all good.

What I have found, and makes sense, is that the skinny string is lighter....travels faster, so it does require a stiffer arrow in most cases.  My spine requirement jumped one size.  Your mileage may vary.

Obviously the point of diminishing returns is always a concern; how skinny is too skinny, how fat is too fat.  Luckily we have folks out there who have already been testing string material for awhile, and I defer to them initially, then see how they work for me.

Skinny is relative also.  If you are shooting a big 70 pound bow with 16 strands of dacron, a 12 strand would seem skinny to you.

As long as you don't cross that line of safety, you should be fine.  You see it's not just, fat or skinny....or light or heavy, there is a medium out there that will work well.  If you like fat strings, by all means use them; same for skinny ones...it's not wrong, just different.

Been following the skinny string threads from the beginning.  Have never been able to get myself to go the 6- or 8-strand route.  They just look scary to me.  However, I have dropped from 14- and 16-strand strings to 10- and 12-strand strings.  No question in my mind they are quieter and seem to make the bow behave ever so slightly better.  Don't know about any differences in speed, and don't care.

The smallest string I have used is a 10 strand df97 with padded loops.  I like that string and have noticed it is quieter and a little quicker.

I just recently aquired a black swan hybrid from vermonster.  It has a skinny endless loop string on it.  I think it is a 6 or 8 strand ultra cam.  WOW!  This is easily the fastest and quietest bow I have shot period.  Until now the quiest bow I have shot was a Mohawk with a 10 strand df97.  I have not ran it through the chrono yet but a 45 pound bow at my 30" draw shooting a 600 grain arrow and not having to change my point of aim but just slightly from 20 to 30 yards is FAST.

Now that being said I don't know if it is the string or the bow.  I think it is a combination of both.  I do know that I'm not gonna change the string like I do on every other  bow I get.  I don't think it can be much better.  YMMV.

Does the drop in strands effect the arrow spine?  Say I go from 18 strand FF to 8 strand of 450?  Will I have to jump up an arrow size?  Anybody thoughts?

Possibly.  It depends on how close you were to the line between arrows before switching out the string.