When you add a specific set of skills to a Sicario, you're going to unlock specific skill bonuses, which depend of the chosen skills.
Those bonuses include:
DEFENSIVE BONUSES:
1. Gradual damage increase (buildings increase their damage during attacks gradually at a % per second of a continuous attack in an area around the sicario for a period of time)
2. Shield (In an area around the sicario, buildings with 0 health are not destroyed for a period of time - After the effect is over, affected buildings remain with 1 health. (Can’t be used more than once during an attack)
3. Lifesteal (buildings regain a % of health on damage being dealt in an area around the sicario)
4. Defense (decreases damage suffered by buildings in an area around the sicario by a %, and is good to be used against missiles and firebombs)
Thing is, the bonuses part is not working yet. So not matter which skills you add to a Sicario right now, they're not going to generate any bonuses until FTX codes in the bonus part...