Compatible lenses Are the lenses from the Canon Rebel 35 mm camera compatible with this digital model?

Question edited for clarity.

You get the Sigma lens for the camera in Canon fit.

https://www.google.com/search?q=sigma+lens+af+75-300+for+canon+rebel+xti

Can you use Sigma lenses on Canon cameras?
Since Sigma lenses are made to be fully compatible with Canon cameras, they are backwards-compatible. This means that you can use a Sigma lens on a Canon DSLR camera. Similarly, the Sigma cameras are backwards-compatible with Canon lenses. Macro adapter loses the AF feature.

https://www.google.com/search?q=sigma+lens+af+75-300+mount+adapter+ring+for+canon+rebel+xti

Sigma makes lenses with a variety of mounts to fit a variety of cameras. If the lens has a Canon mount then it will work with your XS. If has some other mount, Nikon for example, then it will not.

Almost all early Quantaray lenses (and Sigma lenses, Sigma made Quantaray) in the Canon EOS mount will show error messages on Canon digital cameras (but still work fine on older EOS film cameras). usually err99 but err01 also. There is no DIY fix and Sigma ran out of update parts years ago. Current production lenses and Canon made lenses will not have this problem. This lens would only work for someone shooting EOS film cameras with no intention of "upgrading" their system to digital Canon cameras in the future.

Yes.

Yes. Bear in mind that many Canon digital SLRs have a sensor smaller than a frame of 35mm film. This results in the image being cropped, narrowing the field of view and making the lens behave as if it was of a longer focal length.

The Rebel K2 has the Canon EF mount. It is compatible with any EF lenses except the EF-S series. It will also work with third-party lenses that are compatible with the Canon EF series.

You can see the entire current line of Canon lenses (including the EF-S series) at
http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup

Yes, all 35 mm lens should work on canon's digital slr cameras

This lens will work with Canon cameras. I use one on my Canon 30D. It is a fixed F8 you can not use auto or green box because there are not any contacts on the lens to talk to the camera. Its back to the basics manual adjust shutter and ISO to the get a correct exposer. DEFINITLY NOT A POINT AND SHOOT LENS.

All canon EF lenses (and compatibles e.g. Sigma EX) can be used with both analog and digital  EOS-bodies. Depending on the sensor size (usually smaller than a 35 mm negative) a crop-factor due to the smaller field of view must be considered.  Example: Canon EF 50mm/1.8 lens used on a digital EOS (40D, crop factor 1.6) equals a 80mm lens (50mm x 1.6) compared to analog cameras using 35mm film. Some digital EOS-models can also use EF-S lens-series.