Or can they be plugins too?
sequence plugins are for merging and overlaying images, and texture plugins are for procedural. They are writtin in C, i think. But, there is community support here for Python, and there are so many examples to draw from, and plugins are…archaic may be the term; I have not heard of new plugins for years. The world seems to be moving toward Python for extending Blender.
There is no api for importers… one could we written or some easier, just rewrite some file i/o in C, however python scripts can be made fast enough that the extra work for writing in C isnt worthwile.
C is also not as nice as python for reading sloppy ascii files.