HELP!! Restricting Hired unit to certain players

Hi!
Im currently working on my map (TreeTag 4.15). Im not really an expert trigger-maker and ive got a problem. (please note im using reign of chaos, not frozen throne!).
Im trying to restrict a hired unit to certain players. You can hire the unit from shop thats open for everyone, but i don't want everyone to be able to buy the unit. In this case i've made a builder for fire that fire can purchase in the shop, but i dont want the Ents to be able to get the builder too... I don't know how to do this . As far as I know, its something like this:
-------------------------------------------------------------
Trigger: No Fire Builder for Ents
Events:
Unit - A unit owned by Player 1 (Red) (????????) (<- does something))
(+ the other ent players...)

Conditions:
(Unit-type of (???)) Equal to Fire Builder

Actions:
Unit - Kill (???) (<-Class for Fire builder)
Game - Display to (All allies of Player 1 (Red)) the text: WHY WOULD AN
ENT HIRE A FIRE BUILDER?
----------------------------------------------------------------
This is the way ive been learned to do it, but if you have a better way please tell me.. The problem is i don't know how to classify the Fire Builder. "Finishes training a unit" won't work, "Finishes construction" won't work and "Spawns a summoned unit" won't work either. Please help!!!!!!!!!

I dont think thats possible in RoC.. i know theres a Hired Unit function in TFT. Why not just get TFT?

All JASS fuctions work in both, accordign to vex. (even multiboards and the like)

As for how to actually do this in RoC, i have no idea. Go buy TFT if you wanan get in to mapping. Seriously, its less then 20$.

This should be in Triggers & Scripts, so I shall move it.

As for your question, do you understand how to use "Custom script" at all?

Im not familiar with JASS or custom script.. no ;(
But ill check the forums, maybe there is a finished trigger somewhere? :p

I had this needs for my project, and I found this can be done "partially" by training the units, not selling them, and using the functions "limit number of units X for player Y".

Unfortunately, it doesn't work fine sometimes.