Are we Twins? would we make a good couple? who are we lol?

The "Bud-XL" is mostly a Potassium (K) and a bit of Nitrogen (N) additive and wont benefit you much if you fertilize correctly with your base fertilizers! I personally don't recommend using it and sticking with the PK for flowering (don't stop with the Nitrogen output completely but lower it by at-least 50%). PK fertilizers can start being used even from week 2 of flowering (or if shows deficiency of course) and always start with 1/4 of recommended dosage then slowly increase it as the plant progresses (again if deficiency appears-adjust it) *deficiencies cause less stress to your plants and are easier to identify early *During flowering it is not normal for leaves to turn yellow if they do there's a deficiency or even a toxicity!!!! leaves should be green throughout the whole growth! Hope this helps! Good luck and happy growing :)

In the case of boosters, I wouldn't worry about mixing brands. Boosters are almost entirely PK so it shouldn't have any negative effects. And yes, 3rd-6th weeks are the recommendations I see. Keep in mind that if your standard flowering mix is correct, you shouldn't really need a booster. The plants can only take up so much P and K.