every SLC product consists of 3 sub-swaths (IWs)
If your images are taken within the same track (which means they geographically align and have the same extents) there should be no technical problem using them for interferometry.
You can use the TOPSAR Split module to select only one of the three sub-swaths to decrease the file size.
About coherence: It is generally destroyed by vegetation cover. But if you use long wavelengths (ALOS PALSAR) you have higher chances to penetrate some of the smaller vegetation and get coherence from the surfaces. This however ends at high canopies.