Take a look at the back of the tweeters, if there is a capacitor attached to one of the speaker leads in series, then there's no need to worry about a crossover. It will do a high pass filter for you, not the best but that's how 90% of factory component speakers are set up.
When there's an issue on mounting, it's usually not the size that gets you. It's the mounting bracket/surface itself. See those mitsu brackets twist and lock, there are no screws so at the very least you need to make L brackets for them. My crx didn't come with doors, so I can't tell you how hard it would be. It could be as easy as setting them in there and running screws. No idea on that one, but if those tweets have that cap on them then they will be fine. Almost all oem speakers run at 4ohms, when they are amplified like the jbl systems in Toyota, they usually run at 2 ohms.
Dustin - Central Texas