Canon lenses and DSLR question?

Yes. As long as your current lenses are all Canon EF lenses, you should have no problem or cropping using them on the new Canon XSi.

However, the lens that comes with your XSi will not function properly on your EOS Elan. Canon EF-S lenses are made only for Canon SLR bodies with a sensor smaller than 35mm.

Find one brand and stick with it - neither lenses or flashguns are interchangeable between brands. Just about any Nikon lens fits any Nikon camera, Canon EF lenses fit both film and digital SLRs, Canon EF-S lenses only fit their cropped-sensor DSLRs (e.g. a basic T3i accepts EF or EF-S lenses, the EOS 1D and -5D only take EF) . Any Pentax lens made since 1975 will fit a Pentax DSLR, Sony take their own or Minolta AF lenses, and Olympus take 4/3 (not micro 4/3) mount lenses. Third-party lenses have to be bought in a specific fit. Sony, Olympus and Pentax all make cameras that do the same job - Sony's SLTs are better for video. Canon and Nikon are not the entire camera market.

Not sure about Canon, but I had the same question about my Minolta. The answer for me was yes, the AF lenses for my Minolta will fit and work on a digital body - but because the sensor size is different from the film size, the lens lengths will not be exactly the same. IIRC, the salesman told me the lenses would **seem** to be longer; 60mm would behave as if it were a 65mm. Not a problem for most lenses, but your wide-angle specialty lenses won't be quite as wide as before.

Yes, you shouldn't have a problem at all. I use my grandma's SLR Canon lenses on my DSLR Canon all the time.