I guess the mixing problem comes when you need to use mx.* classes, but you're on a pure as3 project.
If it's as3, you can still make an actionscript flex project, using the framework.
When it comes to Flash, you can either export a SWC with your actionscript exported (linkage setup) and add that to your flex project.
Again, if your project weighs more on the flex framework than on flash, you might find the Component Kit handy.
But if it's a Flash IDE more oriented project ( as in simple data coming in, mostly design work, not much nerdy stuff to it ), using a few mx.* classes, just for the HTTP service, doesn't make much sense. The problem comes with the fact that flex is a framework, therefore things rely on each other to work well, like a brick tower...brick upon brick, upon brick...
and since it's code you're working with, things are somewhat dynamic, but still not much cement...so you might end up wasting more time finding all the flex dependencies and getting them to work outside the medium they were made for, rather then building some of the functionality by hand.
If you need an as3 only implementation for WebService, Carlo Alducente has one.
HTH