I know I've seen a few times that you should offer different supplements, but to rotate them, not to mix them. What would the reason be for not mixing them?
1) oversight on the manufacturing process can result in wide variations between batches of the same supplement as well as between brands of the same supplement.. the one study that analyzed a wide variety of the supplements documented that some levels could be as low as 0% of what was reported on the label to several hundred times what was reported on the lable. Mixing them together increases the chance of over and under supplementation due to the above errors.
2) different manufacture processes utilize different particle sizes of materials as well as different stabilizers. Mixing them can cause damage to the stabilization process reducing the stability of the mix as well as changing uptake rates (this can even be seen within a brand for example the instructions on Rep-cal and Herptivite). With widely varying particle size, small particulates can saturate the feeder insect preventing larger particles from adhering.
3) As noted above, changes in competition uptake.. (for example A competes with D3 and E, D3 competes with A and E....).
Ed