Any Advice on this mod

I got a kit from Midwest and want to modify it a little bit to make it a little more stouter. the recipe calles for 6 lbs dark malt extract i added 6 lbs and it calles for 3.3 lbs but i was going to make it 5 lbs and leave every thing elses i got and extra pack of yeast to pitch it after the 2nd week. i read somewhere that it ferments better if you pitch anther yeast if it is over 1.070. i just trying to work my way up to make mine own recipes and i not to sure how to start so i am trying this any advice would be helpful here is the full recipe

12lbs dark malt extract
5lbs gold malt extract
4oz carafa II
8oz Munich
8oz Cara Wheat
4oz Chocolate Rye
4oz Melanoidin
1/2oz northern Brewer
1oz Hallertau Select
1/2tsp Caraway
Wyeast German Ale Activator 1007

A few things:

1) 17lbs of extract is a ton! I get an OG of 1.12 just from the extract, not even the spcialties
2) Get a trial of Beersmith or Promash or beertools. They al have free trials, pick you favorite and buy it. That will help yout get your recipes at least numerically correct
3) If you are really interested in making your own recipes, look into books (Designing Great Beers is a good one)
4) You don't need to buy two yeasts - search for "starters"

ok that sounds like an idea