Re: Reluctant to add formula milk...
Oh, that's great that you're nursing overnight! Those 3-4 nighttime sessions put your baby's intake on a level with what's recommended for a baby his age. 3-4 nursing sessions probably wouldn't really be enough, but 6-8 probably is.
It's tough to find time to express while at work, but if you have the time, and it's worthwhile to you, I'd slip in another pump session or two. Some moms pump in the car on their way to and from work.
As to whether there's a disadvantage of using formula at 11 months... Well, there are a couple I can think of. First, formula costs money. Second, and far more importantly, it's not as healthy as breastmilk. If your only options are cow's milk vs. formula, I'd choose formula until 12 months. If your options are cow's milk vs. breastmilk vs. formula, I'd choose breastmilk.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"