Ok. I'm going to (mostly) disagree with the previous posters.
What I do agree with is that they are adults, and should be treated as such, and also that treating you and your husband with respect is important.
But I don't agree on "just throw the bastards out." Your husband doesn't consider that an option, and it could permanently damage your relationship with them.
First steps first, you and your husband have to get on the same page. The fact that there isn't a single set of rules is causing chaos. Do what you need to come to some basic agreements about how to handle each child. Go away for a night. Go to counseling. Something.
Here are some suggestions:
1) your stepdaughter. I guessed that she was in college. So she's been home for about 2 weeks without a job, correct? Her apartment is too far from her boyfriend? Obviously not a college boyfriend?
I agree that you shouldn't pay for the cell phone in the summer. Or gas.Or anything optional. Car insurance is just silly to have "summer rules" about. however, tell her that you are happy to support her while she's in school, but in the summer you expect her to take care of her non-essential expenses, including her cell bill. If she doesn't give you X by X day of the month (approximately her portion of the family share plan), you'll suspend her service until she does pay up or until she's back in school in the fall. As to the rest, there's nothing like looking at an empty tank of gas and $4 a gallon gas to motivate a job search! Not a punishment, but natural consequences.
2) your son. Yep, he's an 18 year old boy. I'm guessing he has a job, but is not going to college in the fall? Let him know that you appreciate that he's an adult now, but if he wants to live in your house he has to follow certain rules, and that these rules have changed now that he's no longer in high school. Did you previously clean his room? Did you just deal with a disgusting mess in the past? Did he have a curfew, and now you are letting him stay out as long as he texts? Discuss what is different and why.
Assume he's an idiot when you set the rules, and that anything vague is bad. His idea of clean is drastically different than yours, so come up with a mutually agreeable definition. (no really, he may have thought his room was clean enough.) Also, if he leaves food out, don't replace it, let him know you expect him to replace it. Again, natural consequences, not a punishment.
Finally, a really great line I used, almost by accident on my daughter - we had a similar rule about texting when she was out, and I think she still felt we were trying to control her. In frustration one day I said "I do it because I care about your safety. How would you feel if I woke up at 5 Am, had no communication from you, and just went back to sleep?" She never missed a text again. (Also, we had a rule that if we didn't get a text, we would call her at any hour, and keep calling until she answered.)
3) Your stepson. Do remember that even though he was at a house party (bad), he hadn't been drinking (good). What kind of kid is he otherwise? And finally, yelling at his dad like that is not OK, but that is your husband's issue to deal with, not yours. I'm surprised that going home to mom is a viable option, if both your kids are living with you. Is it an empty threat, because he knows it will upset dad? I say, let him go home for the summer if that is what he wants, as long as it really is viable. My guess is that he'll either come crawling back, or he'll stay there and either way things will be better. Again, natural consequences.
Finally, treating your kids as adults includes not just demanding respect, but treating them with respect. Look at your own actions. You talk a lot about yelling and rules and laying down the law. Are you respectful? Kids (and adults) model the behavior around them. Calmly insist on what you need for your own sanity, and pre-discuss consequences with your husband first. Then simply follow through. Don't yell, and don't belittle. if you do it to them, expect them to do it to you.