The generic rule of thumb is to accept coins that have a high volume in trades, everyday.
High volume doesn't always mean that they are real. They might be pumped sometimes by some telegram groups. That's why you have to do your due diligence while looking out for new cryptos.
All coins cannot be accepted because all 100% are not legit. Also there are coins which are not even made for acveptance as currency. Some are utility coins as well
Not always. Take the example of Litecoin and Ethereum.