Is Surety compatible with a Simon XT panel?

Yes, see the page below for how to get started with a Simon:

They would also like a secondary panel in their bedroom and dining room.

There are models of secondary keypad, like the 60-924-3-XT, which should be easy to use, but Interlogix parts are very hard to find unless you purchase used.

Interlogix closed up their security business and all lines were discontinued.