Regarding the issue you have with the tail of Red XIII model, I'm not really sure if the problem comes from Blender. Anyway, a couple of things we could verify, even though it's a bit of "blind guessing":
- Blender can handle n-gon faces, it's something that could screw up during the export. See if the tip of the tail is made only with tris and quads before exporting, that may help.
- Sometimes you may accidentally create two vertices at the same location, and Blender assume these vertices are not shared among the contiguous faces (I hope what I'm saying makes sense). You can verify this if you get into edit mode, try to select the same vertices twice in a row, and if different edges are highlighted on your first and second selection, you have that sort of problems. Two ways of solving it: in Edit mode, vertex selection, press A to select all the vertices, then W ("Special") and in the menu that appears, select "Remove doubles". After this operation you'll have a number of "double vertices" suppressed in this process, in the top bar. Selecting all the vertices and performing the "Remove doubles" operation is something you can do without close examination of your model. However, sometimes making the "Remove doubles" doesn't work (if Blender doesn't recognize the vertices are actually overlapping), so another workaround is to select both problematic vertices and merge them (Alt+A, "Merge at Centre").
That's all I can think about right now. Unfortunately my knowledge is limited to Blender itself, I don't know much about exporting issues except that it's a common source of trouble