Alternate Form already includes the Disguise bonus. From the SRD: "The creature is effectively camouflaged as a creature of its new form, and it gains a +10 bonus on Disguise checks if it uses this ability to create a disguise."
I was under the impression Alternate Form allowed a creature to disguise itself as an average creature of that type, not imitate a specific individual. Isn't it similar to an
alter self spell in that regards, were "you are effectively disguised as an average member of the new form’s race".
e.g. plain vanilla Alternate Form would permit a windigo to disguise itself as an "average human" rather than "Joe the Hunter who mysteriously vanished a few hours ago".
Let's see, Alternate Form says the creature assumes "one or more specific alternate forms"and "the creature is effectively camouflaged as a creature of its new form", but makes no mention as to whether it can duplicate specific individuals.
Besides, we said the Skinstealing allows the windigo to take on the form and physical abilities of the victim, without saying it gives any of the other changes from Alternate Form (size, Disguise, natural armour, natural weapons, extraordinary special attacks).
Hmm, we could make that explicit...
Skinstealing (Ex) #1: If a windigo personally kills a humanoid within one size category of its own size, it can steal that humanoid's skin [as a full-round action?]. A windigo wearing a stolen skin takes on its victim's appearance, voice, size, and physical ability scores in a similar manner to the Alternate Form special ability. The windigo is effectively disguised as the individual whose skin it has stolen (with a +10 bonus on its Disguise checks). It gains none of the other benefits of Alternate Form. A windigo can wear a stolen skin for up to 1 hour, whereupon the skin starts to rip and fray, returning the windigo to its true form. A windigo cannot take off a stolen skin without tearing and ruining it.
However, I prefer keeping the natural armour, weapons, et cetera of Alternative Form, since that's closer to
polymorph which the original description quoted in describing this ability...
Skinstealing (Ex) #2: If a windigo personally kills a humanoid within one size category of its own size, it can steal that humanoid's skin [as a full-round action?]. A windigo wearing a stolen skin takes on its victim's appearance, voice, and physical abilities as per the Alternate Form special ability. The windigo is effectively disguised as the individual whose skin it has stolen (with a +10 bonus on its Disguise checks). A windigo can wear a stolen skin for up to 1 hour, whereupon the skin starts to rip and fray, returning the windigo to its true form. A windigo cannot take off a stolen skin without tearing and ruining it.
Yes, I definitely prefer version #2.