I think iron is taken up by plants very quickly, so when you dose CSM+B the iron won't still be in the water after an hour or so. This is based on reading in the Krib about 4 years ago, so I could be wrong. If that is true, you could dose the micros and macros an hour or so apart with no problems at all. Even if you dose them at the same time, the worst thing that happens is that the iron precipitates out and ends up on the substrate, where it is eventually taken up by the plants. The big problem is mixing the iron with phosphate in a pre-mix. The high concentration of both of them would leave a deposit on the bottom of the bottle containing all or almost all of the iron, and a significant part of the phosphates. Then when you dose the pre-mix, you aren't dosing any iron at all.
I dose one seventh of the EI dosage of all of the ferts every day, with a premix of the macros and one of the micros. I just dose them one after the other, within a minute of each other. As far as I can see it works fine. That is two weak mixes in small doses into a lot of water, so any chemical reactions are far less likely to occur.