Should I stick with 18 credit hours?

Those cles all at once are very standard, and should be fine. The only reluctance I have is that, to be frank, if you are taking cles like that you are still at the nub stages of engineering education so while every engineering student takes 16-18 credits a semester filled with engineering cles, this might seem overwhelming to you this one time because you have not taken any hard cles yet I would think (these are not hard either, but dynamics can often be a weeding cl as a heads up so it takes a lot out of you). If so, just know you will have to get used to it sometime, and to not let it get to you, just try your best. Try not to get overwhelmed, you should be fine if you have figured out how to work hard all ready, if not then this is going to be a hard semester for you, but again: everything will be just fine.