Yeah there's a "rule of 500" which is a rule of thumb for how long your exposures could be before showing star trails. It says that if you divide 500 by the focal length of your lens, you'll then have the time in seconds that should work. IMO use it as a guide and see how you get on. The 1100D has quite large pixels so you might get away with slightly longer.
Here's an example...at 55mm, the focal length is really 88mm because the 1100D has what's called a "crop sensors", i.e. smaller than a full 35mm frame, and this incurs a "crop factor", for Canon APS-C sensors this is 1.6 (55 x 1.6 = 88).
So if we divide 500 by 88, we get 5.68s ... so give it a go at 5 and 6 seconds and see how it looks. The comet is pretty bright, so it should show up quite distinctly compared to the background stars even at that exposure length. A couple of weeks ago I took a couple of shots with my 1100D@ISO1600 and at 15s the Comet was very obvious as a bright green blob with my small telescope (350mm focal length)...admittedly from a dark site...but give it a go!
As for stacking...it's a tough one...I don't know if you'll get hints of the tail from those lengths of exposure