I am about to rent a Canon XF100 for the weekend and need to know if I will be able to import the video into FCE 4?
Possible, but doubtful. Unless you hear from someone who's actually made it work with the XF100, I wouldn't be inclined to risk it.
Canon has two plug-ins: One works for Final Cut Pro 6 and 7 for OS versions up to 10.7 and enables the use of Log & Transfer. There is no mention by Canon about compatibility with FCE, which is ominous. Also everything that Canon says about the plug-in refers to transcoding to Pro Res…no mention of AIC.
The other plug-in works with 10.7 and 10.8 and allows FCP X users to import natively or in Pro Res. A potential work-flow for you would be to download the trial version of FCPX, import the files as Pro Res and transcode to AIC in MPEG Streamclip. Whether you would be comfortable relying on so many moving parts with an unfamiliar piece of editing software is another matter. (I don't think I would be.)
As to your alternative question, your choice is between HDV tape and AVCHD but is probably going to be limited by the rental house's availability. In the tape-based prosumer category, the Canon AH X-1 is still availability in my area and is a decent choice at a reasonable price. In file-based, most of the rentals are high end and don't shoot AVCHD. If your vendor offers a rental for the Canon XA10, it's a nice camera that may meet your needs…or not. Another possibility is to consider a DSLR like a 7D or one of the Rebels. Those shoot h.264 .mov files, which can easily be converted to AIC in MPEG Streamclip.