I have read several different instructions as to how to care for a Crossandra - Sundance plant. Should it be kept in a pot, planted in the ground or kept indoors?
As this is a tender tropical perennial, it is best grown in a container indoors (though it can spend summers outside) unless you live in a suitable climate.
Where are the seeds on the plant?
The seeds will form in the bottom of the flower after the flower drops.
I have a large bed of Crossandra plants; the bed is very crowded. Can I cut the Crossandras all the way back to say 6" and let them start their growing cycle over?
Crossandra is a tropical perennial and most perennials do well with a shearing in late summer or fall. You don't say how tall it is, but I would take off about a third of the height. Six inches might be short this late in the year. Mulch around the base of the plant in case of unexpected cold weather.