hi. now i have 2 questions.
first, is there a way to make player X(1,2,3,4...) selectable? I mean to be computer or human.
second, is there a way to detect a unit's owner type(computer or human)?

In UMS games a player is predetermined by the map as to whether he is computer or human, unfortunately. Because of this you don't need a way to detect it either, since you know beforehand.

What Vrael said kinda implies this, but I'll spell it out: No, you can't make a slot selectable. It's either a comp or a human.

But there's a hack for StarCraft, called Oblivion (just google it) which allows you to swap player slots when you host a game. You can swap a computer with a human and even open a comp slot for a human. But be careful, as the computer is gone then, until you remake.

As for detection which slot is human or computer you can use ai scripts. Human players will ignore most of them. I can't quite remember which can't be applied to humans and its effects easily be detected, but I'd try junkyard dog, random/strategic suicide missions, make these units patrol, arbiter recall, etc. Just try a few and you'll eventually find one.

Very entertaining hack, especially when you spoof "Computer" so people have no idea, keep in mind that when you do this, the maximum number of human players doesn't change, so you can't get a full house.

The only reason I'd say you'd even want to detect if it's a computer or not is to stop that hack, because if a human player is in that slot, like Nude said, AIs won't work on them.

Technically a hack can allow you to do anything in a lobby. This includes making duplicates of players, creating/removing slots, extended races, extended owners, create/remove forces, hide players from view, etc.