It more depends what lenses you are intending to use the teleconverters on as to if they will or will not fit. Canon and Sigma teleconverters have a protruding front element which limits what lenses they will be able to attach to. The canon has far more than the sigma so there are a few more lenses the sigma can fit to that the canon cannot. Further kenko make a highclass range which has almost no front protruding element and so will of course fit even more lenses.
However its not just if it will fit the lens, but if its worth fitting it. Some lenses are not high enough in quality to take a teleconverter and give suitable results for most uses. In addition the magnification factor of the teleconverter determins the amount of quality lost as well as the increase in magnification - the more powerfull the teleoconverter the more it will affect image quality.
There is also light loss and AF to consider
1.4 TC - loses 1 stop of light
2*TC - loses 2 stops of light.
On a canon camera body if the lens setup goes to a max aperture smaller than f5.6 AF will not function unless you are using a 1D series body. Even with 3rd party teleconverter and taping the pins on canon TCs (so that it does not report to the camera electronics) the loss of light is real world and thus will still give a very negative effect on AF - innacurate and slow are the often results.
