I think the categories are ok the way they are now.
What is important is to keep smg, sniper, assault and the others two in different categories, so the player will have to train different kind of soldiers.
For example, one should not be able to switch a soldier that used a smg for 40 missions to a laser rifle or a bolter, without getting penalities.
It is just more fun for the player.... as for the melee combat category, I believe melee weapons are almost never used, with the exception of the stunning rod sometimes, so it doesn't seems a good idea to create a new skill for them.
Eventually, you can keep smg and melee weapons in the same category, and soldiers using the latter will evolve their Strength skill during missions, while the others increase Accuracy, so there will be a difference.