"By the way, do you know whether it's possible to have a negative value for "Supply Amount Used:" for a component so that it would generate supplies independent of the number of stars?"
Umm... supply usages is only invoked when the component is used. The only things that use supplies each turn automatically are active cloaking devices.
"Imperator Fyron, I use your website about modding, very informative. Sometimes there is a lack of information on some very specific topics, but maybe because it is not addressed by MM as well..."
Specific examples?
"EDIT: yes I realized it is becuase these abilities are not implemented, sorry."
"EDIT: In the same subject, about cloaking, when cloaking is inserted as Hull ability, is it always on, or does it need to be activated as well? Am I right to think that in that case it uses no supply?
I looked at P&N mod, and it seems all Hull have gratitic cloak, so it seems that if one doesn't have a gravitic detector, anyship can be invisible?"
Cloaking on ship hulls requires the use of the Cloak order to be activated. Cloaking on unit hulls (eg: mines) is automatically used, and can't be disabled.
If you can detect any type of cloaking on a ship, you can detect the ship. So, if the ship only has gravitic cloaking, all ships can see it, because all ships have EM Passive Scanning level 1 built in (I think it's Passive, could be Active). If you have level 100 cloaking in all areas except EM Passive (no cloaking ability at all for it), any ship can see you without any special comps.

Also, level 1 EM Passive Cloaking is completely pointless.