Gah! Templates are not classes - at least not to my group. They're suggestions and starting points...but not limitations.
It is worth pointing out the statement on YS72: "...while the choices listed here are hardly the only ones available in the Dresdenverse, they represent...best options..." The templates do give us some "musts" - things characters must have to use one of the "pre-packaged character types". That said, I don't think any of the "musts" are prohibitive. The "musts" cover the minimum and the options cover the most common but we all know PCs are unique individuals.
At least that's how I read templates. However, I freely admit D&D has prejudiced me against "classes".
I don't believe I ever stated that characters should be limited to the templates provided in the book. What I *did* say was that custom classes -- especially those dealing with magical knowledge -- should be built to "fit in" with the existing templates. For example, the following should never be a valid template:
TwinkTwinks are those who have, through study or enlightenment, gained the understanding to manipulate all things supernatural.
Musts: None.
Options: Twinks are limited to purchasing powers from the following categories: Creature Features, Faerie Magic, Items of Power, Minor Abilities, Nevernever Powers, Psychic Abilities, Shapeshifting, Speed, Spellcraft, Strength, Toughness, True Faith, Vampirism.
Important Skills: Any
Minimum Refresh Cost: –0
This is obviously an extreme example, but creating a custom template that is basically a variant of a Focused Practitioner but allowing access to full-Wizard specialization is much the same.
@Vairelome: Almost, but the idea was to limit how strong a focus item a caster could create based on their training (as reflected by specializations), since many of the more unbalanced builds rely on the creation of extraordinarily strong focus items. The character could still buy unlimited item slot refinements, and could have a wide variety of weak focus items or a pile of enchanted items, but the limit on focus item strength would in turn make the enchanted items less overpowering.
As to combining templates, this is fine, too. Most of the existing templates have a minimum investment which also serves to prevent abuse. For example, with enough refresh, a Werewolf could develop into a full Wizard. But they would not be able to buy refinements (which is an option for Wizards) until they filled out the rest of the template (ie, finished their basic training, as it were).