As an experienced Portuguese tutor in which 80% of my students are either native speakers of another romance language or already know another romance language, I can tell you that it'd be possible but challenging! 

There are people that can manage to separate the languages in 90% (never 100%), but these a rare students and they make a very big effort and they have a goal not to mix both languages. While the rest of my students simply mix and don't care. I have students that during the conversation uses 70% of the vocabulary in Spanish or Italian and they think they are rocking! 

My question is! Does it bother you to mix languages and at the end of the day not speak any of them fluently and accuratly? 

I learnt Spanish and I'm very perfeccionist, I'm very careful with what I say because I don't wanna go on throwing portuguese words or portuguese grammar structure thinking that I'm mastering it! ;)

I have been learning Italian for about nine months now. I have a fairly good grasp on the grammar, however I still need to learn the vocabulary and idioms etc. Would it be detrimental to my learning process if I picked up Spanish or Portuguese?? I've started learning German this year as well, so perhaps I should focus on Italian and that for a while. What do you think?

There's no hard and fast rule -- if you enjoy it, and have the opportunity to practice both languages, do it -- if you don't enjoy it, don't do it. I recommend paying particular attention to the unique phonologies of the two separate languages, this will help keep them sorted in your mind.