There's no cardinal rule about not dosing macros and micros at once. PMDD formulae do that. However, the PO4 will bind with the chelated iron making it unavailable for plant use. So if you have plants that need more iron then you'd have to dose iron on the off days, so it kind of defeats the purpose of all-in-one, so might as well alternate anyway.
CO2/Metricide will not only not cause problems with ferts, but with dosing, becomes the limiting condition, and if not utilized, can cause algae problems you never had before.
With no dosing and no CO2/Excel, you're limited on multiple fronts, so by limiting light, everything is restricted, but provide more light and dose without a carbon source and you're headed for a world of algae pain.
Just a word to the wise that dosing is not the solution to everything. You can choose to dose or not dose, but dosing has its own set of implications.
Why do you think micros are needed without a carbon source? You're already limiting carbon, NPK. What would micros do for you? Wouldn't that be like saying drink lots of water if you don't have any food?
Edit: Sorry I should have been more clear on this post. Uptake at whatever level would be balanced to prevent algae. So if your light is sufficiently low, you would not get algae. If you need micros at your light level, it implies you need macros and/or carbon, and we all low/fluctuating CO2 causes BBA outbreaks. Low nitrates can cause BGA outbreaks, while low phosphates can cause green spot algae. So the solution in a non CO2, non-macro setting is to reduce the light, not add micros. If you don't need carbon or macros, there is no reason to add micros. Micros improving growth would imply an imbalance to begin with.
if you are using an innert substrate, like many do, and growing plants, they can still suffer from micro nutrient deficiencies. Im not saying you'll get algae if you dont dose micro's and have no other source for those nutrients, the plants will show it.
for set ups with sand substrate, no buffering, you'll be lacking in calcium and magnesium, as our water is often 0 degree hardness, this you will not get from fish food or waste. lack of Calcium will cause poor root growth, and lack of magnesium will cause leaves to go slightly yellow or whitish. In a high light set up these symptoms are very apparent, but in low light without carbon dosing, the problem still exists just not as easily seen.
Even in a low maintnence set up the goal is to have healthy plants, just a much growth rate. Ignoring trace can hamper that.
I am currently using an inert substrate in my 125 (Estes gravel). I never really worry about substrate much after I learned to dose EI, but that's another topic.
I guess it's a matter of semantics. I don't consider dosing Mg and Ca to be trace, as I dose 1 tablespoon of it a week in my 125, which is 1/3 of my nitrates. I dose that as much for osmotic regulation of my fishes (see Rastupus's thread) as for the plants. In those amounts, they are no longer micros. If you look at a bottle Flourish for instance, it's 0.11% Mg and 0.14% Ca. As a matter of fact, there is more chlorine in Flourish (like about 10x) than either of those. So I doubt any amount of trace is going to make up those amounts of Mg and Ca in our waters, which is about 2dKH for both GH and KH the last time I measured.
Well i may have used a poor example, considering how our city water here has zero hardness and most of us compensate for it. But theres more to trace mix's than those two that aren't covered by fish food and waste, iron being amongst them.
When i first entered the hobby (10 gallon days...), i used a sand substrate for planted, no buffering, no carbon source and no ferts. my plants were pitiful, but once i started trace dosing they perked up and coloured up a bit. It was enough to push it into growing slowly and looking healthy. Mind you they didn't look as intense they do now that i use metricide, co2 and full EI dosing, but it still was an improvement compared to what it was.
That may be so, but if you're dosing trace, you might as well dose macros, because the relative cost is actually lower for dosing macros than for trace. I would be willing to bet that the improvements from dosing full EI even without extra carbon at the correct lighting levels would net you much bigger improvements than micros.
My point is not that micros don't work, but that the gain is minimal. The big gains are in CO2/Excel and macros. So if you are not going to do macros and Excel/CO2, why bother with the little gain with micros.
But of course, that's only my personal view. That incremental gain might be what you're after. And if that's the case, by all means, do what you want to achieve.