When I player that mission last week to test it I only had 2 guys escorting Miguel. Miguel was unarmed and I left him proned near the prison gate. My main merc had an UPM40 while Paquito had a Type 59 carbine he I'd gotten from a random encounter. I had around 40 cartridges for the carbine and Paquito had a shotgun as a backup weapon.
I placed my avatar at the edge of the elevated road facing west, aimed-at point so that I would be ready for the first enemy I knew would appear around the corner of the prison wall. I placed Paquito similarly, but a bit further from the gate and aiming at the other side of the passage. It dint take long before they appeared, and with some luck, and choosing the right target at the right time, I got away with only a few scratches. What a glorious battle that was, I wished I could award medal to my troops
The whole friendship/relationship system is very complex, and it has it's flaws. Since I can't do it better I shouldn't really complain, only try to find ways around the most obvious errors.
In this case it's a bit hard to fix. I checked the code and found that if I remove the friendship tie between Miguel and his friends, his friends wont turn hostile if I shoot him the first time I see him. The only thing I can do is make them hostile after he's been killed, and then we have some other things I haven't checked closer, the revenge and ambush back at Sagrada. But I'll continue to try and fix it.