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?

I have never heard any of these. Maaaayabe ばかたち, but I don't think so. I'm sure they're 100% grammatical and occasionally used, but I the short answer is: don't pluralize ばか.

Typically leave it in kana. Non-native speakers tend to use kanji way more than natives do. See a recent discussion on ぜひ／是非 on this forum.

Technically you wouldn't use みんなさん. You would use
1. みんな or
2. みなさん.

There is no みんなさん. I was corrected very often when I used that as a student!

Off topic, but I've never known why Japanese learners so often say みんなさん.

I've heard North Americans, Asians and Europeans use it. Makes me wonder if they actually teach you that.

Besides it being wrong, みんなさん sounds VERY strange to us native speakers.

It is interesting to me this would be surprising to you. Adding さん to human designations is learned early on, so みんなさん seems obviously correct if you don't have a teacher that specifically teaches this (and I can't really remember a textbook that points it out specifically).

Also, it is often difficult for native English speakers in their first years of study to hear the difference between みんな and みな

Took the words right out of my mouth.

What always messes me up is the the fact that every textbook tells you there is no actual plural in Japanese, but when you go to a dictionary they make a dictintion between "much" and "more" or "a little" or "few". If nouns are not countable there shouldn't be such a difference...

I'm not sure why that is significant!
I think "bakadomo" is the most common one there.

To try to confirm this, I tried searching on Google for the three terms and got many more hits for "bakadomo" (in kana or kanji) than "bakatachi" or "bakara". "bakara" actually got mostly false hits, like the Japanese name of Burt Bacharach, so it seems uncommon.