Yeah, I wouldn't worry too much about taking them all at once. I took three at the same time and still managed to get 750+ scores on them.
Also, little known fact: if you sign up for, say, Chemistry and Math II, if you want to do something different when you show up on test day you could take, for example, Bio, US History, and Literature. I had signed up for Math II, but I didn't think I was prepared, so upon arriving I decided to take US History instead.
Oh, and for RDers, December test date is still fine; that's when I took mine and Columbia still received my scores fine.