Yes you can grow hybrids on the beach. It's the best place to grow them. No houses can drop on the hybrids.
Tip the villagers will plant flowers on the beach. Check before you quit to make sure there are no new flowers.
One night I had 3 villagers planting flowers non stop. Got my first blue rose the next day.
Yeah, the beaches are a great spot for flooding with island trees and flowers as there's not much else you can do with the space, so you may as well use it for that.