Generally stay away from any weed n feed products. If you want to kill weeds, use a dedicated herbicide. If you want to fertilize, use a dedicated fertilizer. Generally you do not want to do those at the same time, although September is a little tricky. The rule of thumb for fertilizer is to use it on Memorial Day, Labor Day, and Thanksgiving. The rule of thumb for herbicide is to use it in mid April and, if you really, really need it, again in mid September. Some people think that using a wnf product in September saves time and money, but the problem is the timing. If you fertilize first a couple weeks prior to using the herbicide, the weeds will be growing and healthy. Weeds dies best when they are healthy, so waiting a couple weeks after fertilizing is helpful.

As for the preemergent, YES, NOW. The spring wild flowers are often the product of plants which germinate during fall rainy weather. If you can prevent them now, that will reduce the amount of weeds next spring.