Why do we have these rules for Mikraite?

[alexanderkristall-meth]

We have 5 rules:

(1) keep the Sabbath
(2) dress modestly
(3) no tattoos
(4) keep passover
(5) make the declaration

My question is NOT why are these rules good.

My question is why are some rules of the Old Testament explicitly stated as Mikraite rules? But others aren't?

For example (1) is one of the Ten Commandments. Yet we are not including all the Ten Commandments. Is our reason for not including all the other Commandments that they're obvious? Killing, adultery, stealing are ones I assume we follow but aren't enumerated here.

Is our reason for not enumerating all the regulations elsewhere in the Old Testament that either they're obvious or not followed?

I would assume that everything in the Old Testament falls into one of the following categories for us:

(a) Approved
(b) Rejected
(c) Disputed
(d) Unknown

And in addition:

- Stated
- Unstated

Our current mode of operation seems to be creating a miniature Talmud for practical regulation and everything unstated is handled by intuition.

[fschmidt]

The requirements are meant to be practical criteria for membership.  A requirement like "do not murder (Mikraites)" is not very practical because it won't exclude many people.

[alexanderkristall-meth]

In particular I'm asking about a rule like keep Shabbat.

If we're working with the assumption that people don't follow the 10 Commandments or the Old Testament then it makes sense elect a sub-set that we explicitly follow.

But if we assume a person is already following the Old Testament then isn't this redundant?

My question is what amount of the Old Testament is it assumed we're already following as Mikraites. For example we don't follow the rules about shellfish.