I believe Sankara, Ramanuja et al condoned the idea of animal sacrifice. I think Ramanuja says the sacrificed animal may feel some pain but it would reach a higher state after being sacrificed.
If you know Tamil, there is a u-tube video of a Vaishnavite teacher who says the sacrificed animal may look forward to being reborn as a human in a Vaishnavite household.
(1) Was any sect against it from the time of its founding?
(2) Would rejecting animal sacrifice make a sect "veda-viruddha" since animal sacrifice is very much part of Rig Veda?