Are skills/items like these possible?

1. Revives a fallen ally with 1/4 of HP
2. Causes and enemy to die after 3 turns (50% success rate) and not bosses
3. DEX - 50% 3 turns


Could I cause a Status ailment to change to another ailment?

3. DEX - 50% 3 turns

Simply, make the skill inflict a new State, and make this state to drop the target DEX to 50%, and to last for 3 turns.

1. Revives a fallen ally with 1/4 of HP

That was a option RM2K and 2K3 had, but RMXP not. But you can still do that, in a bit more complex way. Make the Raise skill to have a power of -9999 and to inflict a new State, which reduces the actor MAX HP to 25% and after "0" turns has "100%" chance of releasing. However, that is a kinda werid way to do it, better you will need a script for that.

2. Causes and enemy to die after 3 turns (50% success rate) and not bosses

You will need a script for that, or a really complex event system.

Could I cause a Status ailment to change to another ailment?

You can make a Status ailment to cause or cure another Status. I don't know what do you want to do, but you can make X State to cause Y State, while the Y State removes the X State. And so you will have a state changed into another, the only problem is that the X state will not last. Otherway, you will need some scripting for doing that.