I think the term "Mechromancer" was popularized by the playable character Gaige from Borderlands 2. So it's probably a combination of being cute and of drawing on a verbal similarity to the necromancer (as bradknowles says).

I mean, if you want to get really technical, the "-mancy" suffix originally just referred to divination. So you have necromancy for divining the future by casting bones, or gyromancy for divining through spinning around, or geomancy for divining with rocks and dirt. The meaning later shifted to a more general "doing mystical stuff with" definition.

Probably it would be more thematic to come up with a more science-y term, but "Droid Boss" doesn't have quite the same ring to it.

If you are going to allow a player to do this make sure the player can plan his moves ahead for himself and 5 droids if not the rest of the group will get board.

In an old group i used to play D&D 3.5 with I had a druid, a pet and quite often a set of summons in each battle, this could take up a lot of time if I didn't pre-plan, I could normally get my dice rolls done for myself, pet and at least 3 summons if not more done in the time it took one of the other players to do their move, but at one point I had just over 10 things to play with, so as someone with experience of playing multiple things at once just make sure they do not slow down the game play.

I am just waiting for my new GM to see if he is going to use more than the core books as I want to get my hands on the Special Modifications book