* IB/Zen: yes. IB as broker, Zen-Fire as feed (it is FAR superior than what IB has). Sadly, Zen-FIre does not like that

* Depends. Not for Zen-Fire (you can get the API), and the IB api is public, too, IIRC.
* Only RT.
* Best combination? Hm- depends...
Forget 100/150 contracts per day, that is totally useless as parameter.
The question is mostly what you understand under scalp. How long are your trades? How much delay can you handle? 100 contrats can be 1 lot (100 trades) or 25 lots (4 trades). Scalping can range from minutes to half an hour.... and naturally in that case you dont really care too much about sub second data.
Personally I would go with any Zen-Fire / Rithmic broker - the platform is nice. Ninja is a pretty good manual order entry tool, not so nice for automated strategies. They have some nice semi-automatism, though (they call it advanced trade management - you call the trade, but the robot does things like trailing stops).