I think that it's frowned upon mostly by the Velvet company 
The miraculous benefits of even tension blah, blah, blah...
IMO they're fine to mix and match. At one point on a bass I had Superflex E&A, Anima D and a plain gut G. The Anima D was a nice transition from the steel to the gut. People will often use Olives on the G or D with steel on the lower strings and Animas will achieve the same effect, giving a fatter, less whiney tone on the upper strings. Animas are not very good for arco though.
The miraculous benefits of even tension blah, blah, blah...
IMO they're fine to mix and match. At one point on a bass I had Superflex E&A, Anima D and a plain gut G. The Anima D was a nice transition from the steel to the gut. People will often use Olives on the G or D with steel on the lower strings and Animas will achieve the same effect, giving a fatter, less whiney tone on the upper strings. Animas are not very good for arco though.

Our friend Marco Panascia, who plays with piano phenom Eldar, once mentioned that he couldn't imagine doing the bass acrobatics he does on anything other than Spiros. I'm coming to the same conclusion for my own playing. I just play better on them.