You're welcome. That's what helps for.

No, you wouldn't need to buy another fertilizer for flowering. The plant(s) will more than love the phosphorous and potassium you'll be providing granted the medium is also getting some of the micro nutrients in the wormcastings as well the mycorrhizal fungi from the rootgrow fertilizer.

Roots become particularly vulnerable during flowering as a result of the plant concentrating it's energy on it's flowering process. The mycorrhizal fungi will assist the plant to a near stress fear recuperation in the event that the roots are damaged, and even if that doesn't happen the plant will still benefit from the assisted nutrient and water uptake. As far as what amount to get, get whatever you think you'll need. It shouldn't take much mixed with the medium to get some benefit, though.

The problem with the Phostrogen Plant Food is that it contains more nitrogen than phosphorous, so it wouldn't be very good as a flowering fertilizer. Cannabis generally uses more phosphorous than nitrogen during flowering, so it doesn't require more than what's in the medium already. Phosphorous will promote bud growth and nurture it to it's fullest, whereas nitrogen will promote leaf and general plant mass growth, but not for flowering. Potassium is used to accumulate and trans-locate carbohydrates, the stored food, within the plant. Without adequate Potassium the plant's yield can be severely impeded.

The compost and perlite should do fine, by the way. And if you have anymore questions you know where to come.