Answer to your first post: You should process with your Sonar plugs in my opinion. I like them way better than the effects offered in BFD. I use BFD 2 so it's a little different than Eco. But I have everything set up so each BFD track is a track in Sonar. This way I can compress, eq and do whatever I need to do just like I would if there were a physical wave file there. I have also found that the BFD effects use quite a bit of cpu and I much prefer the ability to use my other plugs like the Sonar plugs, Sonitus, UAD, Waves etc. They just sound better to my ears.
Answer to your second post: No, it doesn't make sense. You should do your bass and drums together if you can. Personally, I do my drums first and then add in my bass. Your drums are your core of the tune. You should always start with them in my opinion. Bass next, then rhythm guitars, then vocals and then back-ups if there will be any, and then filler instruments and special effects last. As you add instruments into the mix, don't mute anything out and don't solo anything up if you can help it unless you are having a problem. Once you get a general curve on your drums, bring the bass in and try to mix them as an entity. You'll be much better off with the kick and the bass working as a team and treated as such.
Hope this helps, good luck brother. :)
-Danny