This is Build 5296, by the way. The leaked build that was making its way around the web last week was 5299. No changes were checked in between the two builds, so they're essentially exactly the same, apart from the fact that this one's an official release.
No, IE and Firefox can coexist on the same machine. They're totally independent of each other.
IE 7 will replace IE 6, but if you need to revert to v6 for any reason, it looks like v7 uninstalls cleanly, so it shouldn't be a problem.
Not sure what you mean by squished fonts. Have a screenshot?
The toolbar layout is a little more inflexible compared to IE6 in terms of customizability, but I think once you get used to it, it feels cleaner having just the back and forward buttons together and everything else separate. Besides, I believe the UI/chrome is still being worked on, so things might change before RTM.
And if you need to uninstall it, go to Add/Remove Programs and make sure the "Show Updates" box is checked.