Technically, yes any size creature can benefit from Alter Self, assuming they have a way to cast it on themselves. It's a personal only spell so it can't be made into a potion, and someone else can't cast it on you. You could make a wand or scroll of it, which would require UMD to use. Also, keep in mind that the gargantuan creature takes a -12 to strength, -6 to con, and gets +6 to dex on top of the adjustments from Alter Self. See the Transmutation section of the Magic chapter in the Core Rule Book.
If by magic resistance you mean spell resistance? No, the creature would lose any abilities that depend on it's form. Generally speaking, I consider spell resistance to be a result of physical form unless granted by class levels or feats or something gained from leveling up, or even items.
Polymorph affects don't change your creature type. You're more wearing a skinsuit that looks like a halfling than actually becoming a halfling. So, if the creature is normally immune to mind affecting things due to it's creature type, it will retain it's creature type and immunity.
The change between forms is instantaneous. The creature will stay changed for a time that depends on what method you use to get there. A wand of alter self would have a caster level of 3 and last for 3 minutes.
Yes, the whole polymorphing thing is very confusing.
When you cast a spell, you know if your enemy has saved successfully.
Succeeding on a Saving Throw: A creature that successfully saves against a spell that has no obvious physical effects feels a hostile force or a tingle, but cannot deduce the exact nature of the attack. Likewise, if a creature's saving throw succeeds against a targeted spell, you sense that the spell has failed. You do not sense when creatures succeed on saves against effect and area spells.
First can an Gargantuan Monster use Alter self to change into a samll creature like a Halfling?
In the Spell i found no restriction about it.
Indeed, it has no restriction. Note, however, that as Alter Self is a Polymorph effect, a Large or larger character alters their stats as part of the spell to conform to their new size.
Second if the Monster has Magic Resistance in its normal form would it have it in the Halfling form too?
Typically, yes.
Third if something changes in Halfling form what is not a Halfling and
its normaly not affected by dominate Person and someone casts dominate person on it. Would that work since its not a person?
Alter self changes the form but not the type / Charachter of the Creature?
Alter Self, as a Polymorph effect, causes you to count as your new form. So, in this circumstance, you count as humanoid for the purpose of humanoid spells.
Four how long does it takes in combat rounds to change the altered form back form Halfing to origianal form?
One, as part of the standard action to dismiss the spell.
Fifth if one cast Dominate on someone and the target makes the save does the target recognize the failed attempt?
People typically recognize that they succeeded on a save, though if the thing they're resisting isn't immediately obvious, they may not know what it was they resisted.
Six if someone has Mindblank and Displacement on itself and the attacker has true seeing then MB would block he true seeing and the Displacemnt would still work correct?
Reading it, yeah, I believe that's the case. True Seeing is trying to gather information about the target (where he truly is), and if Mind Blank can thwart See Invisibility, it should be able to thwart True Seeing as well.
Saethori, alter self definitely doesn't cause you to "count as your new form". That was something that was deliberately changed from 3.5, you now retain your creature type. You do not count as a humanoid.
Saethori, alter self definitely doesn't cause you to "count as your new form". That was something that was deliberately changed from 3.5, you now retain your creature type. You do not count as a humanoid.
Oh? I was looking for it in the Polymorph rules but couldn't find it. Thanks for pointing it out!