Few questions for ranked about shen

I feel like i'm too used to the "Normals" playstyle when I go shen top, where I can be as agressive as I want because in most cases my opponent won't react and their jungler won't come either. However, I haven't had much success in lane in ranked, so I figured i'd go ask for some tips about general and specific things on the forums.

First (this can also apply to most other tanks top lane), when should I start Doran's shield and when should I instead go Cloth-5 (or ruby-2 for the matter, which I do when i'm against AP opponents) ? I find myself never picking Doran's against ennemies that are not almost entirely dependant of AA/only single-target spells (and even there) to do damage, but I feel like that's not the right thing to do... Second : When do you think it's safe to play agressively? Level 6? First item? I usually poke my ennemy laner with Qs when I can and try to focus on farming, however that sometimes lead them to playing agressive during early game (since it doesn't do that much damage and doesn't heal, since I don't necessarly hit them with an AA) and sometimes gets them a kill if i'm not cautious.

Oh, and I just thought about this : Is Darius' passive's damage reduced by doran's shield's passive?

Umm.. That's about it I think. I can't remember what else I wanted to ask. Of course, feel free to give other tips if you have any.

Thanks!