Most plants will stay down without issues in a sandy substrate :)
With some types of stem plants, you may need to use a metal weight (they usually come with the plants) to keep them from floating up while they're establishing roots, but oftentimes they'll be fine if you just get them in deeply enough.
I'm confused with the last question - do you want to keep your plants in pots? What type of fish are you planning to keep in your tank? Unless you're getting bigger fish, or those notorious for digging (some cichlids), you should be able to plant them right in the sandbed with no issues.