Scrambles are limited. I think that 7.1 will allow you to increase the number of scramble missions but you will have to dig into your config file (search for scramble). Scrambles are proximity trigger based and not actual flights with waypoints. Not sure what their performance impact is.
Flights are staggered - i.e. other flights do not take off at the same time that you do. Some have progressed along their waypoints for some time as you take off. There is nothing that stops them from being right overhead as you take off, but odds are admittedly slim.
Creating a player scramble mission is definitely a possibility. it is a very realistic scenario. Lufberry was killed in one and MvR and Boelcke both report jumping into a plane and taking off after enemy AC overhead.
