Okay, I'm not expert, but I'll try to answer your questions as best I can. I've been installing and messing with car stereos since the mid 70's, and I've experimented with a lot of different things. Like line out converters. I've installed them in my cars when I was in my late teens. Hated them! But I was poor and cheap, and they DID the job until I could GET a job.
To begin with, I have never, ever heard of anyone running speakers of different brands and different power handling and different frequency response and different sensitivity ratings in parallel with each other. Sure, it's possible to do, I just haven't seen it done. It’s much better (if not best) to run all matching speakers with matching ratings in parallel. That way, you don’t rob one speaker of something while giving another speaker more of something than it can handle. Just something to think about. But it’s a little late, you already got your speakers.
So now for your line out converter question:
These are about as easy to hook up as a basic speaker is! You pretty much can NOT screw it up! All you have to do is connect your deck’s left speaker output to the left input on the converter and the same for the rights then hook the converter’s RIGHT red output to the amp’s red RIGHT input and the converter’s LEFT white output to the amp’s white LEFT input and you’re done! Oh, sure, then you have to make sure you disconnect your speakers from your stock radio and wire them to the amp’s speaker outputs of course.
The first problem with line output converters is the cheap ones don't do very well with DC-biased head units or amps. If you really want to use one, I would go with the nicer, more expensive ones like Audio Control or at least something that has the amp remote turn-on lead AND has a ground wire of itself. You can read more about this at
http://www.bcae1.com/loc.htm
The problem with line output converters is sound quality more often than not really sucks. Seriously. Your stock deck will start to distort way before your amp and speakers start to come alive! I have never seen a line output connection in any car that sounded very good above a low volume. Most stock car stereos put out maybe around 10 watts per channel or so (max). So, when you crank up the volume, you’re going to be up in the end of your rms rating of the stereo before your amp really sings.
Personally, I never recommend line out converters to anyone. Especially because of how cheap you can pick up brand new aftermarket decks with RCA outputs. You only need one pair technically for a 2-channel amp. Heck, you can find good working decks with RCA outputs on craigslist all day long for under $20. But since you paid $180 just for your 6x9’s, it seems money is not an issue. In that case, you can always pick up a decent aftermarket head unit with RCA outputs brand new for under $30 at Walmart that have RCA outputs. It would be a much better choice than running a line out converter. But for now, go ahead and hook it all up and let us know how much hissing and distortion you hear. Because line out converters will help your amp increase whatever sound comes into it from your deck.
Hope this helps, but it probably won't. Let's wait to see what other members have to say. I have very little experience with line out converters only because I don't suggest them or use them myself (since the 70's). Again, I'm not expert on car audio but I've installed more decks and amps and speakers than I could begin to count, using these two books as my "Bibles":
Just to give you an idea of a little of my past experience, this is my last car's sound system:
http://s254.photobucket.com/user/Hairball98498/library/98%20Stratus%20stereo