I have not tested using these coupons all at the same time, but here are some links to some good pet coupons:
$5 off $25 at Petco (expires 2/15):
$5 off of Science Diet Petco coupon:
$5 manufacturer's coupon for Science Diet:
There were also Science Diet coupons in a recent Sunday insert.
Petco should allow you to use both their coupon and a manufacturers coupon at the same time. However, they might not allow you to use two Petco coupons in the same transactions. In other words, both the Petco Science Diet Coupon and the Petco $5 off $25 coupon.
No Petco, no worries. Petsmart ususally excepts competitor's coupons.