Ive been wondering how you use these poisins like tremble. Do you put it on a throwing weapon or a bladed mele weapon? Does anyone know?

I would assume they work much like poison in Oblivion. They are just applied to whatever weapon you have equipped. Possibly can only be put onto melee weapons (all thrown items count as melee; even grenades O.o).

In your recipe book effects are listed just buff or debuff according to recipe. have yet to see a npc affected by this stuff yet so prob forget crafting, its pretty useless unless your PC is hardcoe, may matter more then. But if paint gun is in game I wpould assume dart gun is possible. rockit louncher is.

Grenades aren't under explosives?

The blast and damage is, but the distance/speed of the throw is based on Melee.

Rocket launchers and grenade launchers use Guns for their accuracy and part of their damage, but the blast of the explosives themselves are stronger (and maybe bigger if you take a certain perk) go off the explosives skill.

It's all very confusing :/

I believe the entire dart gun schematic was removed from g.e.c.k.

ALL FO3 craftables are in the GECK. But the recipes for Dart Gun, Deathclaw Gauntlet, Railway Rifle, Nuka-Grenade are not in-game. you could add them with the console though I think.