More than happy to answer any questions. We monitor any Alarm.com compatible systems (Simon XT and 2GIG included). Let me address your questions in order that you posed them. (A summary of which features are included and/or available in the different packages can be found here)
Activity Monitoring: All of the sensors installed on your system are “security monitored”, meaning they can trigger alarms and cause the police to be dispatched. Sensor activity monitoring is an additional Alarm.com service that goes above and beyond security monitoring to let you see the status of specified “activity monitored” sensors through Alarm.com.
Sensor activity monitoring is limited to 10 with Alarm.com’s Gold Interactive because “sensor activity monitoring” dramatically increases the cell phone signal usage of the alarm system. It sends a signal to Alarm.com every time that sensor opens/closes or changes in any way.
Keep in mind that not all sensors can be activity monitored. To understand this, remember that your smoke alarm is either activated or not activated, regardless of the status of your security system. Same for Carbon Monoxide detectors, etc. Activity monitoring allows you to see if your motion detector has been activated recently, or whether a door has been opened, or a window is currently open, etc. even when the system is unarmed. As such I do not feel that all sensors that can be activity monitored are worth activity monitoring. For example I don’t see a real benefit being derived from using activity monitoring on my glass breaks, but I’d pretty interested to see if the motion detector in my office is activated after the dog walker stops by (with no reason to explore that part of my home). But everyone uses their systems differently.
You can add additional sensors to be activity monitored for $2/month for each 5 additional sensors. This is a pass through charge.
Video Monitoring: You can add levels of video monitoring. For $5/month, the basic video package supports up to 4 cameras and has (a very small) 50MBs of storage. As currently designed, its most useful for checking in live. But if you would like to add the next level of video monitoring, it is an additional $5/month and supports up to 8 cameras and 250MB of storage. Each additional 250MB on top of that is another $5/month as well.
Arming schedules: This allows you to set schedules for automatic arming and disarming. We’ve found this feature thus far to be used almost entirely by businesses who do not want to give all employees codes for the alarm. The use of notifications, geo-fence reminders, temporary user codes, and remote arming and disarming make arming schedules unnecessary for most other users. I believe what you would be interested in for the thermostats is smart energy management.
Smart Energy Management: This thread gives some detail on the smart energy management add on – http://suretydiy.com/forums/topic/whats-alarm-com-smart-energy-management/