There are stimulators- they are called Auxins. The thing is - there are many different kinds.
As for P with Fe- I asked the same question- they used to NOT put them in the formula together- they do now, and have not had any problems to date (they've been doing it I THINK they said for 6 years now) before that they did not, but when they wan test, they say found no problems with it. More than anything though- it works for me!
I wonder if they're able to mix them because they make small batches that are fresh- Does it take a long time for them to react with each other? Does that make sense? But they said it still has a good shelf life (though it does have to be refridgerated...