Of course a supervisor is still good, but would you need one if it's just airsofts?
Yes, wise thing to do is to have a singular intelligent person manage any weapon props. Armorer is definitely the best if you can afford it. When fake guns look like real guns and the moron grip brings his real gun that get's mixed in with the batch, well, it's bad. Sounds like an unlikely situation, but still. Handle prop weapons just like real ones. Check to see if they're loaded etc, hand it to the actor, take it away as soon as scene is done. Even if someone put a BB in there, don't want to shoot your actor in the face with it and score a nice big welp for the rest of the shots haha. That's much more likely to happen. Goofball grip plays with it between sets, leaves it loaded with bb's.
We use airsoft quite a bit. I always have to talk to extra's about how to use them, and usually we're shooting somewhere (legally) where cars drive by or has some foot traffic so you have to explain that you can't play with the gun. We usually make them put it on the ground between takes. Even with all that, we always end up with a guy start playing with it at some point that we have to politely remind that the passerby's can't tell it's a fake at 30 feet.
Another thing about airsofts is that the barrell has a much smaller hole in it than barrel or 9mm or more. Will audiences see this in shots of actors holding guns on actors and keeping it still?
Cheap guns maybe, the more expensive metal guns are a straight replica. You get what you pay for. Even the expensive guns are much less and much safer than the actual firearm.
If it's too much money out of my range, then I can do fights with knives,...
Knife fights generally require tons more CG than gunfights. Most blades in movies nowdays are 3D because actors generally don't like being stabbed, even with a fake blade. Besides, where would the fake blade go when filming a stab? And no actor is good enough to throw a knife and hit a target, let alone with people around. Gunfight CG however is usually totally doable in 2D compositing software.
My friends said they would not find a fight exciting if it's not a gun fight, and that knives and clubs just don't cut it in the action genre. But I think they can be, if done right, but I could be wrong.
That's nuts, some of the coolest action scenes are knives and fists (can't remember one with a club, but several with baseball bats). In fact, I'd venture to say that knife fights are scarier or at least make you cringe more. Getting shot sounds like a much less painful way to go haha. Regardless, it's a mix or good choreography with good actors and stunt men, good cg, and most importantly good directing that makes an action scene. Directing action well is hard. You want to let the audience know what is happening all around while directing focus on one aspect/action/character at a time.