There are must be a lot of apps that are designed to communicate with server. My question is only about App installed on iOS device + Server side service interaction. Web app is
Well that depends on what you know, there is reason to choice one of the other
This would reflect on your first question, you pick the language on you needs. Thus if python makes it easier then pick that one.
Again not one that is easy to answer, since it all has to do with requirements. But any SQL server will do. Cassandra is meant for "scalability and high availability without compromising performance" accourding to there website. Do you think you webservice will get many request then it might be a choice to consider.
This again is only going to be answered when you pick the a SQL server and server language.
Start with something simpler, you are really going out on a limb here.
Propably there is some, but you should really start small and work from there.
Twitter started out as a Rub on Rails app and is working on scalability and availability which ruby is not really good ar (that is my person opinion). or Look at facebook they have written a php to c compiler to make php run faster.
The only thing I can say to start code, when you app does take off then tackle the some of the performance issues.
And since you state that you are new to programming do not bite of more then you can chew.