A: You can file dependent I-485's as long as your case, the primary one, is still pending. There is a risk in adding your wife later. You are taking the following risks:
1. Your green card could be approved BEFORE September. If this is the case, then you would need to file family-based green cards for all your family members and wait for their family-based priority dates to become current. This is a very small risk, as we doubt that your green card will be approved within 2 to 3 months, but it is a risk you should be aware of.
2. Priority dates could retrogress between July and September. If that is the case, then the USCIS will reject your family's cases, as the priority date would not be current at that time.