For simplicity's sake, this inaccurate and partially wrong graph i did by hand is visually what happens with bacteria and food/nutrients/whatever you wanna call it.
Bacteria grows, starves, creates food through dead bacteria (breakdown of organisms creating ammonia), bacteria bloom because of extra food (Ammonia -> No2 -> No3) then dies to starvation (no more ammonia to break down) and this all repeats until the bacteria to food ratio is established and then your tank is properly cycled.
Now, I just made this graph and explained a little about bacterial life cycles for fun. It really doesn't matter when you add Po4 into the equation.
As far as the relationship of Po4 to cycle specific bacteria during the cycle process, well there is none. On a normally cycled tank without a shrimp, Po4 isn't present until there is a breakdown of No2 to No3 and your tank starts to accumulate Po4 through the breakdown of No3 and waste in the tank. Even then in a basic tank cycle your not going to be noticing Po4 until you start feeding your tank with flake food known for adding Po4 into your tank. Or, directly leached from dead rock or other items which is the only place you would see Po4 in the tank come from during a cycle. I'm no biologist but im pretty sure bacteria are simple job specific organisms. They aren't like us that eat anything on our plate. Specific bacteria break down annonia to No2, and then another from No2 to No3.
IF Po4 was a direct food source for a beneficial bacteria that we want plenty of in our tank then we would be using that instead of GFO to remove it. Unfortunately Po4 is a delicacy of nuisance algae which like to quickly take over our tank if our Po4 isn't in check. Algae is also job/food specific which is why reducing your Po4 levels typically kills off things like hair algae. Actually, contained algae growth is also known for reducing Po4 in reef tanks as seen with che
ato balls in your sump or other fast grow algae methods.
So, to sum it all up. You should use GFO/Po4 remover always including your cycle as there are other issues aside from the no3 breakdown that can introduce Po4 into your tank, things like uncured LR, older sand, improperly filtered water or leaching through other means. Po4 is and always will be a bad thing in a reef tank, you should focus on it even during your cycle. There are other things that can produce high levels during a tank cycle other than waste break down and cause you to have a higher than normal Po4 coming out of your cycle.
The only reason you would want Po4 in your system during a cycle is if you are prepping for a planted FW tank, or are setting up a marine algae tank or mangrove garden.