Would one or two mammals among our ranks be too out of place? For example I'm thinking of strapping an Engine of the Gods to a Mumak of Harad instead. Thoughts?
If you are aiming for strictly fluffy then it wouldn't fit with the lore of old world. However, this is the Age of Sigmar. If you want a Mumak EotGs then make one! Here are some mammals @Aginor has included in his army.
On the old world nope but this is a new dawn, skinks have tamed numerous beasts over the centuries and every Slann has a memory of that event. Go nuts and post pics
Seems unlikely. A stormcast is not even a proper human as they violate several qualifications to be a mammal, in fact they aren’t even properly living. Provided there are maybe a few weird War beasts, so I say go for it!
So actually, there is Haunchi the Old One who takes the form of a jaguar. Those blessed by Haunchi were capable of passing through thick forests with ease. There is even the Jaguar Standard that gave a regiment the swiftness a jaguar to hunt down their enemies. I've heard of others incorporating jaguar mounts into their armies rather than Cold Ones, but I haven't seen a picture of it that I recall. I say go for it!