[RPG] attune a Circlet of Human Perfection to the animated skeletons to allow them to blend in and speak

The spell animate dead usually applies to the targeted corpse the template skeleton or the template zombie

There's probably not going to be any direct developer support for or against your GM's ruling, likely because the developers considered the actual rules so obvious there was no reason to make such a ruling.

The spell animate dead targets one or more corpses, not specifically humanoid corpses and not mandating the resultant creatures be, for example, humanoid skeletons. Further,

• A zombie can be created only from a mostly intact corpse.
• A skeleton can be created only from a mostly intact corpse or skeleton.

The spell says nothing about transforming a corpse from the corpse of the original creature into the corpse of a humanoid. The template skeleton provides several examples of nonhuman skeletons, and the template zombie does the same. Seriously, all evidence points to, for example, a mostly intact 9 Hit Die green dragon corpse becoming either a 9 Hit Die green dragon skeleton or a 9 Hit Die green dragon zombie when such a corpse is the target of the spell animate dead (and requiring the expenditure of 225 gp of onyx as material components).

So, while it's hilarious to imagine, for example, a wizard casting the spell animate dead on the mostly intact skeleton of a vicious advanced megaraptor and ending up not with an advanced megaraptor skeleton but a 1 Hit Die humanoid skeleton, or a wizard casting the spell animate dead on the mostly intact corpse of a crocodile and ending up not with a zombie crocodile but a generic humanoid zombie, that's also deeply weird.

By the way, prior to this question, I'd never considered any other way of interpreting the spell animate dead except to animate monster corpses as monster corpses, but, as evidenced by this 2010 EN World thread, at least one other GM uses a similar house rule to your GM's house rule. (Or maybe that question's also about your GM's animate dead house rule? [Cue dramatic music.])

Working around these limits anyway

However, even if the GM insists that the spell animate dead when used to make skeletons or zombies only creates 1 Hit Die humanoid skeletons or zombies, options remain. The spell animate dead can turn a big ol' pile of those 1 Hit Die humanoid skeletons or zombies into a necrocraft, or the spell can make an isitoq or a beheaded. It will be difficult for the GM to argue against these alternatives given such creatures' descriptions' increased specificity.

These alternatives are just samples. The more bestiaries available, the more options multiply.

Yes, familiars are creatures and, as such, can attune to magic items so long as they meet the requirements for attunement (For example, the magic item cannot require the attuner to be of a certain class, and the DM must deem the familiar capable of wearing/attuning to the particular piece of equipment).

However, some additional restrictions may apply. For example, if you are playing in D&D Adventurer's League, you are subject to additional restrictions as per the latest DDAL FAQ:

Can my familiar attune to a magic item?

Any item attuned to a creature under your control (familiars, beast companions, etc.) counts against both your character’s limit of three attuned items and the character’s permanent magic item count.

This rule doesn’t imply that such creatures have the ability to attune to magic items. Whether or not a mindless undead creature— for example— can attune to something is subject to DM discretion.

[DDAL FAQ v8.1, p.9]

As this is an official restriction for sanctioned play, it is not unheard of for such restrictions to also be used in home games.