I am evaluate your product and it seems good.
Here are some the questions I got.
When the queue is enabled and send sms to queue, where is the queue created? in sim card or PC hard drive? How large this queue can be?
There is mention in the document that messagingToolKit support loadbalance, which class should I use? and how to use it?
The queue is created in PC. A folder is created per channel to store the message in XML file. The file is removed after sending successfully.
For load balancing support, have a look at the BulkGateway sample code. It can do loadbalancing using round robin or by the number prefix.