To start with, welcome to RF!!! Great to have you.
Now on to your tank and your questions. I will start by saying that it's never a good idea to put any type of Anemone in a tank as young as yours. They need a well established tank, 6-12 months. Your choice of a Sebae may end up biting you in the behind. They're one of the hardest anemones to keep in captivity. They're also pretty light demanding. The lighting you have on your tank really isn't adequate for any anemone. If you're wanting to stick with Compact Fluorescent lighting, I'd look for a fixture in the 2X65 watt range. That would still not really be enough lighting for anemones though, however, a lot of people, including myself, have had good luck keeping Bubble Tip Anemones in that lighting. Mine are Rose Bubble Tips.
Next, You currently have 1 Occellaris and 1 Yellow Tail Damsel. That Damsel will quickly grow. It will probably become too large for that tank. As it grows, it's going to get mean!! With those 2 fish, you'll soon be overloading your biological filtration. Adding more fish would be disastrous, IMO. If I were you, I'd try and trade the damsel to an LFS for credit. If you did that, you'd be able to put another Occellaris in your tank. I wouldn't recommend adding a Clarkii Clown though. Other than Maroon Clowns, Clarkiis are about the meanest buggers in the Clown family. I can almost guarantee that it would kill your Occellaris, which is one of the least aggressive in the family.
Carpet Anemones get HUGE. A 55 really isn't even large enough to house one. Having 2 different types of Anemones in your tank is asking for trouble. People with very large tanks are sometimes able to do it...but not always.
I'd also recommend not adding anything right away. With a tank that's only 3-4 weeks old, it's not even finished cycling yet. Also, do some research into the pros and cons (mostly cons) of deep sand beds.
I'd recommend getting rid of the damsel, adding another clown of the same species. Maybe taking the Sebae back and waiting to get an anemone until your tank is more established and you're able to upgrade your lighting. In a 29, you could have a couple clowns and probably only 1 more fish. A 6 Lined Wrasse would be good...or any number of fish from the Blenny or Goby family.
Sorry this isn't the advice you were hoping to hear. Do some good research into the needs of each fish, coral or invert before purchasing. It'll save you a lot of heart ache in the long run. Again, welcome to RF. We have a great bunch of friendly and knowledgable people here!!