Defector Unit

[SkedarE]

Barney's infiltration into a resourceful CP position may have been attained through hacking, manipulation or secret evasion of combine mechanical or psychological brainwashing practises. It may have involved technology the likes of Alyx's combine hacking device/combine over-world code USB to tamper with combine routine records, recognising Barney as an individual already having been through standard CP indoctrination Processes.

This process or technique of infiltrating, considering you only see Barney in CP uniform and with CP codes ect. ,sounds a little too difficult to allow a cheap spammable unit for the rebels. If included, unit advantages would be minimal at best over any other unit, being poorly armed and a weak unit. The unit would take possibly the time needed for the combine to build a heavy vehicle like an APC or strider and would require certain unique abilities to balance its role against cheap spammable infantry ect.

Abilities may include opening/commandeering enemy(combine) gates, temporarily/permanently shutting down/reprogramming defence turrets or hopper mines. It could also be used to replace the rebel engineer, as it would be more fitting for a unit, with previous experience in the realms of combine codes and combine technology in general, to deploy combine hopper mines and turrets for the rebels.

The combine defector units already exist as CP's and the combine would have standard uniforms and identification as a requisite for being enlisted in it's services, so rebel modals would be unusable for combine units. Combine over-watch are bio-mechanically enhanced, altered and brainwashed into the Combine service. They are will-less drones for the Combine and essentially a human state no longer applies to them. Combine Over-watch soldiers are a different species. Thus defecting Combine Over-watch soldiers would play into a completely unlikely and implausible scenario in the Half-Life universe.

[Tristan]

I think by brainwash they mean, filling his mind with propaganda and lies, but because Barney was a Rebel he knew it was propaganda and lies. He probably told them what they wanted to hear like, "All hail the overseer!" or some garbage and went on to his "lower than average" beatings. He said "Im behind on my beatings today when they came knocking on his door. So probably the combine want you to beat twenty innocent civilians or something to death or you get shot. That would explain the why they kill people and don't shoot their boss.

[SkedarE]

I think by brainwash they mean, filling his mind with propaganda and lies, but because Barney was a Rebel he knew it was propaganda and lies. He probably told them what they wanted to hear like, "All hail the overseer!" or some garbage and went on to his "lower than average" beatings. He said "Im behind on my beatings today when they came knocking on his door. So probably the combine want you to beat twenty innocent civilians or something to death or you get shot. That would explain the why they kill people and don't shoot their boss.

'Barney; " I'm way behind on my beating quota. "' Yes. Civil Protection are volunteers that are allowed free will in their services. It is likely propaganda would have been one of few low level psychological buffers to encourage CPs to follow Combine commands or directives. CPs are the most optimal unit for combine to rebel affiliation conversion due to this.

However, as was previously stated, Combine Overwatch Soldiers cannot be "converted". Dr. Breen and Dr. Mossman aside, individuals are offered a higher rank and more combine synthetic enhancements at the expense retaining less and less free will. Those "elite" Combine Overwatch Units, donning white armour suits and a single, circular red visor, are for all intensive purposes synth walking, talking and shooting robots. The grey armoured Combine Soldiers we're more familiar with would not likely be in a very different state, less skill in combat and fewer Combine synthetic augmentations aside.

[Surrealistik]

Now that we have the Saboteur apparently filling this niche, you might consider giving him the ability to disguise per the Red Alert 2 & 3 spy:

http://cnc.wikia.com/wiki/Spy_%28Red_Alert_2%29

Basically, you select the ability, then select an enemy infantry target; the Saboteur's appearance and team colour changes to the targeted infantry. While disguised in this way, the Saboteur counts as a unit of the targeted unit's owner for the purposes of autotargeting and minimap colouration. Whenever the Saboteur performs a hostile action, his disguise is lost and his Disguise ability goes on cooldown for some period of time.

It might also have (or gain via research) an assassinate ability that allows it to instantly kill any single infantry unit in melee; naturally losing its disguise as above.