Are 仙草, 毒草 and 良草 broadly-used idioms?
Sometimes you can, just like you can use pot to refer cannabis when you talk to other smokers, you have to make sure the listeners are 网络小说 addict, otherwise they would totally have no idea what you are talking about.

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I don't read 网络小说, but I know some US TV drama fans call drama 草, looks like all addict call their favorite 草. :)  

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Here is a list translated from tieba

仙草：epic

粮草：good

干草：so so, 

水草：bad

带毒干草：even bad

毒草：terrible, unreadable

穿肠毒草：Call doctor, need 仙草 to save life