I'd agree you just use ばか as the only plural I can think of would be ばかたち and that just sounds strange for some reason.
Also I thought when addressing a group of people you would use みんなさん
rather than あなたたち
I dunno, as people have said, plurals seem to not belong or be needed much anywhere in Japanese. Then again what do I know, only been studying a little while
another random thing is ばか usually put in kanji as 馬鹿 or left as katakana for caps-like-emphasis?