Is it possible to combine the two items "Amulet of Mighty Fists" and "Amulet of Nayural Armor" so that you can get both the bonus of both items but in a single item? And if that is possible is it possible to remove either the natural weapon or unarmed attack bonus from the AoMF and still get the AoNA bonus?
And my second question: is it possible to be a small creature when that creature is normally limited to medium only?
Here's the key component for your question:
If the item is one that occupies a specific place on a character's body, the cost of adding any additional ability to that item increases by 50%. For example, if a character adds the power to confer invisibility to her ring of protection +2, the cost of adding this ability is the same as for creating a ring of invisibility multiplied by 1.5.
So per RAW, you take the cost of one item, and add 50% of the cost of the second (or more) items you want to add as effects. So if you want an Amulet of Natural Armor +2, and an Amulet of Mighty Fists +2, you take the cost of one, and add 50% of the other (usually the cheaper one should be increased by 50%).
Some GMs either will not allow it, or will run this differently. Some GMs may allow you to 50% up the cheaper of the two, others require that you pick one of the two costs, usually the one that is being added to the existing item. It's important you confer with the GM as to how he runs it, so you know what you're getting into.
I don't understand your second question.
Only if your GM lets you.
Correct, you can only be a different size through GM FIAT. It's not wrong to rule that an adolescent creature is going to be one size smaller than it's adult size. For example, the Young Template here would give you want you want. Ask the GM if you could get that added to your character.
Darksol, the way you phrased your answer sounds like you add 1/2 the cost of the second item. This is incorrect. You add the cost of the second item multiplied by 1.5.
Example: +2 Amulet of Mighty Fists and +2 Amulet of Natural Armor = 28k (16k and 8k = 16k + 8k*1.5 = 16k + 12k = 28k gp).
The way you stated it is that it would be 20k (16k + 8k*0.5 = 16k + 4k = 20k).
Thank you for your explanation on my first question
For my second question say I was a nagaji which are normally medium creatures could I instead be a small nagaji?
For the cost of 2,500 gp plus spellcasting fees you can have the Reduce Person version of Permanency cast on your character. Note that this is subject to removal should you be targeted with Dispel Magic, so plan appropriately.