Occam 说:
First Aid 10+4 will help your companions but if you get knocked out it won't help you. You'll only get the heal from whatever First Aid skill the companion left standing with the next highest has...so 10, at best.
It's still useful to heal you in between the "sub-battles"; when the amount of soldiers is real great, and the battle is done in several stages, you get healed in between.
Maximus, yes, I almost always use 4 skillers and 4-6 fighters. Prisonners can be nice for money, I know many people use them that way, but in my games, money is never really a problem. In early game, I fill Calradia with enterprises, and then, denars just flow. That way, it's kind of a pity to use skillpoints in Prisonner Management.
Hmm to be honest, your question about 120 sea raiders really depends too much on my and my companions lvl, on difficulty settings, on the type of soldiers... If you really want to, you could solo them: run in circles around them (clockwise) at larger distance, let them run out of missiles, then circle closer to them and take them out 1 by 1 using a long range 2h weapon. But I wouldn't have the patience

I like to play safe, I hate to get knocked out and let autocalc decimate my precious troops, it boundlessly frustrates me. I'd typically take no less than 25% or 33% of the enemy's amount of troops; would make the balance too bad.