First let me say, iknow nothing about fly fishing.
I've been fishing, essentially, crappie gear for trout for 40+ years. Micro reels, 2# line, #10 hooks with dough.
Mostly fish the waters of Cherokee Reservation. Kinda rednecking it... No waders just waling the river, stringer tied to a belt loop.
So, anyway, I just bought my first "fly" rig. A 14' tenkara. Something to play with, not get dinner.
I'm really looking forward to hooking a 18" rainbow in it.
Will the casting work if I use a #10 and a couple BBB?

There shouldn't be any reason to add weight to get a Tenkara rod to cast. Use weighted line sized to the rod and you can cast flies or a baited hook.