I'm assuming the traditional sized caboose you already have is the Lionel no.19752, in the first picture below, which would be the ideal size for what you're running, it sounds like. For an additional caboose, there are a numbe of choices. In the second picture is the Lionel Northeast style caboose. This is a scale caboose, but isn't too large. The main difference from the Lionel traditional bay window is that the scale cabooses are a bit wider. The bay window in the next picture is an Atlas O Trainman caboose. These are scale sized as well, so, again, will be a bit wider than much of the traditional sized rolling stock. Both of the these cabooses are on sale at Mr. Muffin's trains, and other places as well. I think the K-Line caboose (next picture) you mentioned would be fine as well, about in the same size as the scale ones, perhaps just a bit smaller. I have several K-Line bay windows, and I think they'd work fine with traditional rolling stock.
Below that are the MTH 30-77167, MTH 20-91647, and MTH 30-77307. They'd all be about the same as the other cabooses mentioned above. Lots of choices, although they may not all be available, so look around. If you already have a line on a K-Line bay window, I'm sure that would work fine for you.