There was a study conducted in China, students learning both English and Russian vs students just learning English had the same level of English comprehension at the end. Go for it.

I don't know. When I stalled at higher intermediate level in Spanish, I decided to try learning Portuguese. I will say that as a foreign learner of Spanish, becoming a foreign learner of Portuguese is both fascinating and confusing. The two languages are sufficiently similar that my brain is not doing a good job of distinguishing them, and I accidentally get them mixed up when speaking--both when speaking Spanish to Spanish language companions, and Portuguese to Portuguese language companions. I don't know if this is such a bad thing, but it is definitely happening to me.

Polyglot or Bust!