If iron reacts with phosphate does that mean you can only dose one or the other? Could I not buy a stabiliser and add that into the mix? Thanks for the advice and is ferric phosphate harmful?
No, you can dose both. Most people often dose micronutrients on one day, and then macronutrients on another, alternating between the two.
The concentrations at which iron and phosphate are at inside the aquarium are insufficient for insoluble iron phosphate to form. The problem arises in concentrated solutions and/or storing them in dry powder form.
I am unaware of how you could introduce a stabilizer into the mixture. Rather than doing that, it would be easier just to buy the chemicals separately, and make two bottles of solutions and/or store them separately (dry).
Ferric phosphate is not harmful, but is insoluble, and results in the nutrients being unavailable to plants.