Surrealism  

I'm not an art expert but to me, surrealism is a painting style rather than restricted to a certain time perid so in my opinion it would be more than fine to group bosch with the surrealists, as he is one. You could group him on his own and take the view that he is not part of the surrealist movement, but since when has art classification been that rigid? Just my opinion :D

Here's webster's take on surrealists or surrealism "the principles, ideals, or practice of producing fantastic or incongruous imagery or effects in art, literature, film, or theater by means of unnatural juxtapositions and combinations." Additionally, surrealistic is defined as "having a strange dreamlike atmosphere or quality."

Bosch was before surrealism, perhaps even one of the first, right???

Yep, I agree surrealism is a style that shouldn't be restricted by time period. Heck, what about current surrealists? There's alot of great stuff now being created out there, though now they're more likely to be grouped with the current outsider art movement, which has a wide umbrella.

safe? nothing in art is safe, unless you're a folk artist.

surrealism isn't restricted to a time period, it's a style so yes you can group him in there