Jim said it would be very bad because wizards live a long time, and like to record information (maybe by the BAT?).
By that logic, shouldn't White Court Vampires—who speak freaking Etruscan, are functionally immortal, and love to hold on to information for blackmail purposes—be just as poor candidates?
As to the OP question, I think there might be something there. There's a lot that we don't know about Maggie. It's possible that Malcolm found out some things, but I have my doubts about him being a Venatorum. (Singular should be Venatorum, as it's just Latin for "hunter," and I presume it's genderless, but I have trouble remembering the ludicrous number of different declensions in Latin as I get older). In my opinion, Malcolm got roped into things by Maggie, and I think she'd pull the classic Wizard routine and withhold a lot of information to protect him (which never works, and it's why distrusting wizards is such a popular trope).
Funny that Harry never thought to ask Ivy about his mom; he was almost willing to trade what amounted to his soul for information from Chauncey, but won't ask the creepy little girl who knows everything. Maybe it'll occur to Harry to do so in Peace Talks.