Beastie Bloomz, Cha Ching, & Open Seaseme are not sweeteners. They are "high-phosphorus fertilizer" according to the company. Basically they are bulking boosters that do different things at different parts of the plants life, but none of those things are sweeten as far I have seen.
You can use both Bud Candy & Flora Nectar from the start of flower. I would suggest continuing use the bud candy through your flushing period as well. This will help prevent weight loss, in fact it can actually help to boost weight. In addition to that, you should look into using some Molasses during your flush.
I would also suggest you start low with the dosage of Bud Candy & Flora Nectar and raise the levels to full strength as the plants get further into flower and they can actually use the amount of sugars and carbohydrates you are giving them.
Using all of these things together shouldn't be a problem as long you pay attention to the plant and don't overdose anything.