Hi Rook,
Plenty of + countable noun is correct as you said (eg "plenty of options, plenty of places to go" etc).
Plenty of + uncountable noun is also correct (eg "I have plenty of time", "He has plenty of friends" etc).
"There was plenty of people" is not correct, you should say "there were plenty of people" (because "people" is plural).
No, we don't usually say "plenty of" in negative sentences. There are many different quantifiers that you can use for negative phrases like "I haven't got a lot of ideas" or "I have very few ideas" etc.
If you have any questions, please let me know.
Jamie (www.italki.com/jamie.teacher)