I am building an Aspected Magician as an NPC, and I came across an oddity with Mentor Spirits: Many of the Magician bonuses for Mentor Spirits are either casting-specific or conjuring-specific.
To work around this and make Mentor Spirits way more useful for Aspected Magicians (and slightly more flexible for full Magicians), I plan to use this house rule:
The Magician bonuses for Mentor Spirits that apply to "spells, preparations, and rituals" can alternatively apply to summoning the appropriate category of Spirit, according to the Magician's tradition (e.g. Combat Spirits for Dragonslayer, which would be Fire spirits for Hermetics or Beast Spirits for Shamans). Conversely, Magician bonuses which apply to summoning a specific kind of Spirit can alternatively be applied to the spells, preparations, and rituals of the appropriate category (e.g. Water Spells for Sea, which would be Illusion for Hermetics and Detection for Shamans.) This is an either/or choice, the Magician doesn't get both bonuses.
(Rat's a bit of a special case, since its bonus is strictly for Alchemy. For completeness I'd allow Rat the choice of either bonus to Man, following the above rules.)