we should create new thread or something for every flag
Technically yes. But for identical issues you can create one thread along the lines of "Here's a bunch of ponzi scams: thread1, thread2, ...." and then use it as reference on multiple flags.
Can we use a community thread for flagging potential scammers? I ask because it says you can create 1 thread if you tag flag many users, or can this be a simple thread that states I flag people for these reasons and leave it at that.
Yes, but make sure that if someone goes there, it's clear what the flag is about.
Scammer flags should usually each have distinct topics.