I think, if you want to go for a "beach" scene, use the white sand with the tan rocks and the first or third cave. Be careful with buying them online, because the size could be too big for a smaller tank. The plants are plastic and I'm not a fan of. I bought a plant very similar to your last one and used it for a bit in Edwin's section. It torn his fins up, nothing horrible, but I wasn't happy. So I bought a silk plant from PetCo (http://www.petco.com/product/109231/...ium-Plant.aspx
) and one from Petsmart (http://www.petsmart.com/product/inde...mdn=Fish+Decor
- it's a lot greener in real life).
The second one was too long to fit in the tank I wanted it for, so I cut some leaves off the ends and stuck them in some shot glasses and put them in the main tank. As for the plastic plant, I took them out of the base, stuffed them in a shot glass, turned the glass upside down and stuck the first silk plant on top if it in the back of the tank. Basically, don't be afraid to make modifications to your plants/decor!
Here's some pictures and more info: http://www.bettafish.com/showthread.php?t=64709
I'm sure whatever you decide will be fantastic and your betta(s) with love it.