Recently switched to flats..
 

A few months ago, I took the B string off of my Spector 5 string (I hardly ever use it anymore) and since I didn't have a high C, I've had it strung EADGG. I use the two G strings occasionally for a cool effect. Anyways, today I decided to put some D'addario Chromes (flats) on my bass. I bought the 4 string set, and I recently ordered a high C string to complete it. My question is, is it alright to keep my old stainless steel G string on the bass with the Chromes until my C string arrives? Will this affect the neck in any negative way?

The neck will give you the answer. I can't see it being an issue, however you may need to adjust the truss rod when you get all the strings on.