Some doodad and trigger questions.

In level 6 human (i think its this one) campaign, the doodad houses are destructible, i want to ask, how can you make them destructible? If i find those house doodads and place them, they cant be attacked and stuff.

And, how can i make units completely transparent? I've made some wisps, and put the trigger to set them to 100 percent transparency, and they are still the same color and visible! Thanks.

With WEE Blink! you can use doodad models on units. (use mpq viewer to find model paths). So using WEE Blink!, take a building, and change it's model to a house.

As for your transparent unit, I guess you can use the Line of sight blocker (its an invisible doodad) model in the same way.

All the doodad houses are also listed in "Neutral Passive" and those are destructible. :)

(I'm not at home right now so i can't check) But are they all there? Like, the pub, all town houses, colors, etc

yup the houses are also units in neutral passive, i believe it was under cityscape. You should check it out..
also, if you want a doodad to be killable, just use a random unit, and place the doodad you prefer as a special effect over the unit. I think the special effect will be killed with the unit, but if it doesn't, just use a trigger saying "destroy special effect" on the unit killed event. However, with the Enhanced World Editor you can specify the model of the unit as a path, you can also use doodad models for them (just the path of the special effect, but without the extension). That's probably the easiest way if you want to make killable doodads.