1. What are your targeted levels of phosphate, nitrate, and iron?
CO2 range 25-35ppm
NO3 range 5-30ppm (KNO3)
K+ range 10-30ppm (K2SO4 or GH booster)
PO4 range 1.0-3.0 ppm (KH2PO4)
Fe 0.2-0.5ppm or higher (?) (Plantex CSM +B)
GH range 3 degrees ~ 50ppm or higher (GH Booster)
2. Do you, and if so how much, modify the PPS pro recipe to increase/decrease phosphate/nitrate/iron?
I don't use PPS pro. Instead, I use EI dosing. IMO PPS limits PO4 because it was once considered to cause algae. It does not. Outdated? Many still use it. You decide. I simply prefer to dose all nutrients at non-limiting levels. It's the logical approach.
3. Are the people recommending 4+ ppm of phosphate correct?
Nothing wrong with it at all. Many tanks don't need levels that high but it won't hurt anything.
4. What is, if such a ratio exists, the recommended ratio of nitrate to phosphate?
Ratios are for farmers fertilizing 1,000 acres at a time. In the planted tank forget the ratios completely. It has no direct influence on plant growth. Supply all nutrients at non-limiting levels regardless of ratios. Check out Liebig's law of the minimum
. Here is a post
I made explaining the concept of non-limiting nutrients in the planted tank.
5. Is the iron/phosphate precipitate occurrence true? Would that occur in the aquarium or only when the two substances are put in the same bottle?
Iron and phosphate will precipitate if the concentrations are high enough. Generally, you can dose them on the same day without problems because the volume of the water in the tank decreases the concentrations lower than are required for significant precipitation to occur. You cannot however mix them in the same bottle due to the high concentrations.