You will need a sub, a nice mid, and a high quality tweeter/great set of components before a super tweeter should even come into the picture. Even then depending on what you listen to, they may not even be needed. Start with a nice set of components, (if you don't have at least $1,000 into speakers, then you don't need super tweeters you need something else). Check the frequency range of your components and compare it to what the super tweeters offer. What are you listening to, that you think these might even be a good choice? Right off the bat, if you are not sound deadened, then I would do that before I started trying to get a very limited amount of high pitch.
Depending on what you have so far for your system, you may want to get a set of components and "Bi-amp" your components instead of running a crossover. Bi-amp is running each speaker off a separate channel from the amp. So you are running four channels to one set of components, instead of two channels and splitting it half way down the line with a crossover. If you can match the amps selectable frequency range to your speakers frequency range, you are only running power through those frequencies that your speakers will use. Instead of pushing power through the entire range of the amp, and then cutting off the frequencies when it gets to the crossover. Its using your amplifier more efficiently.
The 6.5"s will fit right in the door. The 8"s will fit under the seat, unless you plan on fabbing something custom. I would go with 6.5 just because you have more options. You can get 5 1/4 mids too.
As for quality it really depends on your budget. I got a deal on some JL audio ZR650s a while back, and love em. I bought two more sets just to have because they are discontinued. What do you have so far for you system, and what is your budget?