Not everything is available at the moment in spark but over time more and more mx functionality will end up in the spark set. FB2 is much better at dealing with the mixing of mx and spark and this will keep improving as bugs are ironed out
<?xml version="1.0" encoding="utf-8"?>
xmlns:mx="library://ns.adobe.com/flex/halo" layout="absolute" width="400" height="300">
the above is the basic mx module it still will allow you to mix spark and mx, so yes you can use mix and match, the caveat being that at the moment you may run into some issues but thats what testing is all about. If you have any problems you can always put your code up for others to look at, if it is a compatibility issue then it can be listed as a bug.