Sparta Mod

I'm familiar with XML for civ mods, as I'm experienced with modding buildings and tile improvements. However, I've never made a civ before. I've read some guides so I know how to use Lua code a little bit, and I understand the syntax. I got an idea to make a mod adding Sparta to the game using Whoward's mix and match modding. The idea I had was to make (one of) Sparta's UU a Helot which is a Worker than can only be acquired from city-state tribute, but is half the influence penalty and is gotten instead of the usual Worker that city-states give, and that instantly finishes improvements in 1 turn if on the same tile as a Spartan combat unit. The issue with this, however, is that this does not replace an already existing unit, and I'd be fine if this was done either through a new unit or simply adding a promotion to the Worker that can only be acquired if gifted by city-states, but how would I do this, and would I use XML or Lua? I'd be fine if someone either told me how or copy pasted code for this. I'm not sure how to make a special unit only gifted from city-states or a promotion that is gotten only when gifted from city-states, and I'm also not sure how to make a unit have faster tile improvement speed when on the same tile as a friendly unit.