Question about elemental resistance/penetration

Hey guys
I always have found confusion in forum and I'm still not sure about the questions I'm going to say.
Let's say we have fire penetration gem (-38% fire resistance) in our main skill and we use dual curse flammability and elemental weakness (for another total of -90%) if we are going to hit a mob that has 0% fire resistance this mob goes to -128% fire resistance or there is a limit such as -75% resistance?
My other question is: penetration is really important if we fight against a boss or other mobs that have +50% or more resistance. So we need to lower that at least at 0%. But is really important as a concern the damage to put down the resistance of a mob to -128% too? Or the damage we are going to gain is few, so we just need to face a mob with -60% resistance.
I hope someone knows precisely the answer :)
Thanks
Regards

http://pathofexile.gamepedia.com/Resistance_penetration

1) i have never heard of a cap on negative resists.

2) The higher a mobs resistance, the greater relative benefit you receive from penetration and reduced resistances.
With some map mods, mobs can have over capped resistances though. In those situations it is possible a curse may not have any noticeable effect. Penetration will always work though.

Not 100% sure but almost: Penetration ignores the overcapping of resistances whereas reduction doesn't.

Ex: The cap is at 75%, but the the mob is overcapped at 150%.
- A penetration of 30% would make you deal damage as if the mob had a resistance of 45%.
- A reduction of 30% would affect the overcapped amount, reducing the overcap to 120%.

They stack additively, so against a mob with 75% resistance not overcapped, with 30% reduction and 30% penetration you deal damage as if the mob had 15% resistance.

And what about values above maximum elemental resistance for ex: Fire 75% (145%) is there any additional elemental dmg reduction from it ?