My overnight sour dough went slack, could my 'starter' have eaten too much gluten?
There is no way to answer all you questions definitively. 
I can say your conversion most probably off because cups are not a precise way to measure flour by weight. 

First and foremost if you want to bake bread on a regular basis learn to use the bakers percentages and the metric system. Buy a quality kitchen scale. 

bakers percentages (BP) are what professional bakers use and for good reason. Dependable, repeatable results. You can scale up or down a formula with precision. 

Basically the BP system is based on the amount flour always being 100%  in the formula  and all other ingredients are a percentage of that. For example a bread dough that has a hydration level ( amount of liquid in the dough ) of 70%  

1000g. of flour so a hydration level of 70% would be 
700g. of water   - 1000 x .70 = 700

Every ingredients percentage is multiplied into the amount of flour.
Salt is generally around 2%  

1000 x .02  = 20g

Once you learn this system you can change the amount of flour and calculate all the other ingredients with precision. double it, triple it, cut by 67% whatever you want. 
Once you understand it you can develop your own formulas, you can backward engineer recipes in cups with the help of some conversion charts and math and/or your scale.
if you go crazy like me you can make an excel spread sheet so all you have to do is enter a number of loaves you want and it will calculate the whole formula for you. 


  WE DO NOT KNOW THE HYDRATION OF YOUR STARTER.
  lets say it was 100% -
  150g flour to 150g of water =300g


your formula called for 

flour 100%--   1 kilogram   (1000g) of flour 

water  60%--   600g water  60%  

salt   18%--   18g salt     (1000 x .018 = 18g )  18%

starter 30%--  300g starter (1000 x .30  = 300g ) 30%

Adding the starter changes things but because we assumed it 50%  it is not much.
starter and preferment's complicate the BP system but its all math it can be figured out, i will not go into here. 
your formula shows a overall hydration level with starter at 65.2% that should be a rather firm dough not wet and slack as you described. thus my saying your conversion was off. Also changing the salt % as you did will have effects on the leavening time. ( salt is an inhibitor, slows down yeast or starters )

Sourdoughs or starters or yeast do not "eat gluten"  they feed on the sugars and enzymes in the flour.  I will let you do your homework on that. Gluten is developed partly by this process and partly by kneading. 

Learning to do a "window pane" test will help you to know when gluten is developed enough. I suspect that is what you meant by "create a 'skin', a tight skin, for making this artisan type loaf". Learn what the autolyze process is.

You "mixed it at 9:30 PM, and attempted to make it into loaves at 10:30 AM" That is  long time to proof at room temps and would allow the leavening agents to exhaust themselves.  did you proof in the fridge? Sourdough breads are more temperamental and finicky then commercial yeast breads and you have to be able to recognize when to bake them, it is easy to let them to far.

A sourdough mother can be any hydration you want and it will behave and taste differently depending that hydration level. You have feed it on a regular basis, you pull some of it make a starter, sponge, preferment, always feed/replenish it. NEVER feed it more than 50% of its total weight. lets say it is a 100% hydration mother and you have 1000 grams after pulling some to build a sponge, you would then feed it 250g water and 250g flour = 500g which is 50% of its weight. 

A healthy mother will be forgiving, you let it go for a while and bring back to life with a few feedings, you freeze it for a while. 

Have fun and enjoy a fresh loaf of bread soon.