Harry's Pentacle is more of an aspect than a Focus Item.
Making light with it is an invoke of the aspect to counter a "darkness" scene aspect or somesuch.
Using it to kill a loup garou, invoke for +2, also the narrative explanation for satisfying the catch.
Tracking Thomas, using it as a symbolic link.
one could go on...
As for Focus items.
I let all casters create their own focus items for their spellcraft. Otherwise you cut off a lot from the casting powers.
Enchanted items are another matter. Imho pure Evocators and Channellers can not create enchanted items of any kind.
Thaumaturgy Users obviously can create the whole range of items.
Thematically limited Ritualists can create all kinds of items, blocks, attacks, maneuvers... but they need to fit the Theme.
Ritualists limited to certain fields can only create enchanted items if they are specialized towards Crafting.
A Ritual(Wards) user can not create enchanted items nor can a Ritual(Divination) User.
A Ritual(Crafting) user on the other hand can create the whole range of enchanted tems, without limit to effect or theme, just like a full thaumaturge.
As for changing Focus items. I let them change those together with enchanted items, at a significant milestone.