Not only that you will invest in multiple coins but in different investments as well like stocks, bonds, real estate and even in gold. That way, if you lose in crypto, you will never experience such huge loss as you still have chances to make profits from your other investments. And yes, invest only on the amount you can manage to lose, although you can spend some of your life's savings but take only a portion of it and leave the rest for your other needs as well.
Yes, invest also outside the crypto market, much better in tangible assets like real-estate or gold.
Because if other fail, you have your tangible asset that you can see and hold.
Crypto market is still a very volatile one, so investing here is not an assurance that you will get your profits.
Those simple rules as stated by OP are easy to apply but some are ignoring them because they are after for high profits.