Is there a way to resize blood splat decals?
Yes, edit the particle.
Is there a way to increase the firing rate for weapons?
Yes, edit the animation, or alternatively, cut the animation off earlier in the weapons script.
Is there a way to make particle effects that are created by weapons loop forever?
You cannot make particles loop forever unless of course you just spawned another upon death.
Is there a way to have more then 1 firing animation for weapons such as if I press the primary fire button you get one animation but if I press the alternate fire button ( the scope enable/disable button ) you get another firing animation? Haven't we answered this?

From my understanding, I think this is what you want. I had a mag 60 with explosive secondary and bullet primary fire on left and right mouse click lol its a killer weapon but its been awhile though since I was focused on it xD.

This would be a script function you need to do a script, search for this through the scripts to understand.

if((self.Owner().Buttons() & BC_ALTFIRE) != 0)

Its actually applied to the ridingGun.txt in scripts folder that will help to understand some I believe look for ALTFIRE in that document.

Is there a way to change a monsters skin texture when it dies like if it's killed by the flamethrower it turns black?
I haven't looked into the monster code, but you could potentially to a similar thing to what they do with limbs.
Is there a way to make the fire that gets on a monster when it's burned by the flamethrower to cover it's whole body and not just it's upper torso?
Unless it's a part of the monster's animations, edit the particle.
Is it possible to have a monster spawn another monster through scripts?
ActorFactory.Spawn(), lookit the API.