Yeah you need to follow the recommended dosing though. For NPX it's 100mL per 25 gallons. And they suggest starting with only 1/2 that to get started so that the possible bacterial bloom won't rob all the oxygen and crash your system.
What I found in my own case was that it took a little bit for the pellets to get started. I was running GFO
as well as Purigen, and the flow was a bit too high. I ran it that way for three months, and I saw little if any change in nitrates. My phosphate was great, but way too low and became a limiting factor.
There appears to be two types of bacteria to culture, and as you attempt to do so on the solid carbon it may help to carbon dose (vodka,vinegar,sugar) with the small amount of NPX in order to jump start the bacteria in the tank, and possibly encourage them to proliferate into the water column where they will be taken up into your fluidized reactor.
Keep us up to date with your progress, we like to learn from each other...