Is a language of some deciders decidable?
I don't fully understand the details of your question (especially the notation that you've used).

However, the language you're asking about appears to be finite (something like the language of descriptions of all Turing machines that halt for all inputs and have at most 100 states), and every finite language is decidable.  You could, in principle, produce a list of all the strings in your language, and then just design a Turing machine that accepts exactly that list of strings.

Note also that reducing $L$ to the halting problem wouldn't prove that $L$ is undecidable, since every recursively enumerable language reduces to the halting problem. You'd just be showing "If I could solve the halting problem, I could solve $L$, too." That's a bit like saying, "If I was the world's strongest man, I could lift an apple." Maybe you don't need to be so strong to do such a simple thing? If you want to prove that $L$ is undecidable by reductions, you need to reduce an undecidable language to $L$: then, you're saying, "If I could decide $L$, I'd be able to decide this undecidable language, and I know I can't do that."