Yes it is possible to become Phosphate limited, not very common, but it can happen.
Using somehting as simple as Sodium bi-Phosphate (normal aquarium pH down) will suffice, you dont need to use much, and the added sodium wont mess up your ionic balance as long as your doing regular water changes.
I suspect that most nitrate issues (when there are no phosphates) is coming from overfeeding, and no enough siphoning and removing of watse, and possibly, underskimming.
Again, getting rid of every bit of waste as you can will eliminate this cause (excess stored waste causing nutrient cycling) and allow you to identify exactly what the cause of your high nitrates is. You can then correct it, and generally, once you elimnate the cause of the high nitrate, you system will usually come back to balance, and normal carbon dosing will suffice to remove nutrient by itself.
For instance, in the systems i was previously nitrate limited in, i am no longer nitrate limited in. With a regular feeding regime, and good husbandry, i have found the balance between nitrate and phosphate:organic carbon, and no longer have to compensate.
having said this, from where the systems started (they were in a pretty bad state) to now, has been nearly 10 months.