Mixing Supplements - Will the Distributions Remain?
It's hard to maintain homogeneity for the mix of powders. The bigger the batch, the less homogeneous it's going to be. Ideally you want to disperse solid ingredients; there are several techniques to do that. Two of the most popular ones are:


dissolving all the components in a non-reactive solvent and they dry out or precipitate the powder;
using a ball mill.


With supplements, however, it's somewhat trickier, as they may react with each other over time, with the solvent, or be contaminated in grinder. Also they are often granulated for a good reason: this reduces the contact with atmosphere and prolongs shelf life.

If you decide to avoid the dispersion part, then due to Brazil nut effect the largest powder particles will remain on top of the batch, the smallest – on the bottom. This is especially noticeable when the batch has been shaken, e.g. after being transported in a car trunk, or just being kicked a few times lying on the kitchen's floor.

All in all, I wouldn't recommend doing this. The probability of suffering from an imbalanced diet is too high. There are some multivitamin and mineral complexes designed for humans, maybe look for something similar for your pet.