Hmm. After staring at it for a minute, you have a pretty good trade-off here. You only have to get 2 farseers total rather than 3, you can mix & match guardian types (which is way more fun than spamming 3 of the same thing), and you get mini-obsec. On the other hand, losing out on the 3+ casting of a "real" seer council can hurt!
I think this would be a reasonable set of rules to try out.
That said, I second HappyJew's suggestion to let the guardians reroll to-hit rolls of 1 (in shooting and melee respectively). Back in the day, Ulthwe's Black Guardians were meant to be a cut above the rest, and this was represented with higher
WS/
BS. Boosting their
WS/
BS even higher than it is now for "free" would probably be a bit much, but letting them reroll 1s is a nice, toned-down little bonus that I wouldn't mind getting in exchange for the lack of seer council super-casting.
I give it a thumbs up!