Hello New2this.
When you say that the plant is in flower for 1 week- do you mean that the plant has shown sex (preflowers) or you have actual flowers growing? There is a difference. Either way you can still top at 1 week, but it will slow your growth considerably as the plant uses its energy to heal the cut. But, if you're growing outdoors, it won't matter as much as if growing indoors as you have no height constraints outdoors. You might want to bend her over a little, but that too will slow growth- the result of either method is to create more bud sites as opposed to one main cola. Ultimately, you'll end up with pretty much the same amount of product at harvest whether you top or not. So why slow things down now? The plant is genetically only capable of producing so much- by improving environment we can help the plant get closer to it's full potential, so.....
As far as your fertilizer goes, you need to look at the NPK ratio of the MG stick. The nitrogen (N) should be lower and the phosphorous (P) should be higher for the needs of a flowering plant.
Good luck to you and let us know how it works out.