This is not a true explanation because I have had situations there my Si has rise to very high concentrations without creating any diatom bloom. But it have been in tanks with measurable PO4 concentrations - not in a new tank with no/low PO4. Si concentrations in a normal reef tank is around 100 - 200 µg/L because of leaks from stones. I have had around 17 000 µg/L Si without any diatom bloom. ICP from Triton Lab
IMO - its a myth that diatom blooms is driven by Si concentrations in the tank water - they are instead - IMO - driven by low PO4 concentrations that outcompetes other organisms for space because the others not get enough of P in order to grow fast. When it is measurable PO4 in the water - the green algae will outcompete the diatoms for space because higher growth-rate.
But they are not difficult to be rid of - snails, urchins, hermits and Bristletooth tangs are good diatom predators.
About time and size of the CUC. As early and many as possible is my answer as soon as you switch the light on. If you wait until you see some algae - you've already lost the battle. Golden rule - if you switch on the light - introduce a sufficiently large and varied CUC - do not wait. There is food - even if you can´t spot it.
Sincerely Lasse