Need help with Gluten development

I was going to make some Italian bread loaves this A.M. using KA AP flour but my wife had used most of it for pies (go figure).  I used AP for the preferment yesterday, and I have KA Bread Flour on hand, but I've learned I like the softer crumb I get with the AP flour so I chose to use (In what I thought was a flash of brilliance) some White Lily light all purpose flour (essentially cake flour...I think it's about 8% protein).  I probably had about 1/3 KA AP and 2/3 cake flour.  First thing I noticed was how brilliantly white it was (forgot it was bleached), then after mixing for a reasonable time I realized how weak the dough was as it failed the windowpane test badly and wasn't as smooth as I'm used to.  Now I tried to decide what was the best fix, add bread flour (I didn't want to make it much stiffer and I was trying to avoid increasing all ingredients proportionately), or vital wheat gluten which I had on hand.  I decided to add wheat gluten, about 1 Tablespoon from the freezer, but it didn't seem to make much difference right away (I don't know if it should work immediately, or needed an autolyse, or much more mixing or what), so I then added about 2-3 Tablespoons of bread flour and continued mixing.  This time the dough looks like it should, and windowpanes much better.  It's fermenting now.  I'll let you know later how it turns out.  I'm just wondering what you guys would have done.  Thanks for the feedback.