The lenses marked EF-S are lenses that share the same mount as your EF lenses, but the lenses themselves are smaller. The amount of light that passes through EF-S lens is lower due to its smaller size, and there isn't enough light to project light over something as large as 35 mm film. Why? The lens is too small for it and only meant to project light onto the smaller digital sensors used in almost all DSLRs (eg: your Canon 350D).
For example, if you mount the current Canon 18-55 mm EF-S kit lens onto a Canon 5D or your Elan, it'll definitely mount fine because it has an EF mount. The problem isn't that it won't mount. The problem is that you'll get large amounts of vignetting, since the lens isn't large enough to cover the film.
Um.
No.
All of Canon's film SLRs (in the EOS series) use the EF mount. So too do the EOS 1Ds series, the EOS 1D series, the EOS 5D, and the EOS 10D. The other EOS cameras (300D, 350D, 400D, 20D, and 30D) use the EF-S mount.
A camera with an EF-S mount point will accept EF lenses. A camera with an EF mount will
not accept EF-S lenses. They physically
will not mount. This is because EF-S specifies a mirror size that is smaller than that allowed for EF, which in turn means that an EF-S lens can have elements that protrude further back than an EF lens.
if you do manage to mount an EF-S lens on an EF body, it is liable to collide with the mirror as it swings up, causing damage to the lens, the body, or both. It can be done safely on the 10D if you modify the lens, but Canon officially discourages such a practice.
If you have a Canon film SLR, and you're planning on getting a 5D, probably the best wide-angle lens to get is the 17-40mm f/4L. It'll mount on any EOS body, and is excellent value for money. The 16-35mm f/2.8L, I'd only recommend if you have a need for the faster glass. It won't be particularly wide on a crop body (one that accepts EF-S glass), but it'll do a fair job for the interim.
The rest - about the reduced image circle, and so forth - is correct, though.