Could I apply to just a couple schools now and later add more schools down the line after e-submitting? Any disadvantages to this? And, how do I add more schools after I e-submit?
Yep! I did that last year. You just go back to the school designation page like you did the first time and then pay.
As for disadvantages, I can't really think of any unless you're adding schools in like November. Then again, I added MWU AZ in late October and was still accepted in December