1) I like playing with 6. It's not as easy to bluff, but boy is it fun when it works.
2) I don't play for money, but I have a lot of friends who do. A typical weekly game here in Fairbanks, Alaska has a $20 buy-in. To make it more exciting, going up to $40-50 USD seems good.
3) They should play Texas Hold 'Em. The rules tend to get distorted in most casual groups, but I think a general guideline is that you start with the person to the dealer's left contributing a "small blind" of 50 cents to a dollar, and the next person to the left contributes a "big blind" to the pot. Then the dealer deals out 2 cards, face down, to each player, after which betting commences with the player to the left of the one who contributed the big blind. This player can choose to "check," meaning they postpone betting to see how everyone else bets, or they can raise (make a bet), or--if their hand is terrible, like a 2 and a seven in different suits--they can fold, although people shouldn't usually fold before the first few cards are flipped up by the dealer.
Betting progresses clockwise around the table, and after the first player bets no one else is allowed to check. Any player who checked prior to the first bet gets the option to meet the high bet and stay in. If a player does not want to meet the high bet, they fold.
After the round is done, the dealer "burns" one card (puts it face-down beside the deck), then flips up three cards in the middle of the table that count as part of everyone's hand (this is the "flop"). Players bet again, following the same rules as before. When this round of betting ends, the dealer burns another card and then flips up one more (the "turn"). After another round of betting, the dealer burns again and flips up the final card, called the "river." There's one last round of betting, then all remaining players either show their hand or fold, going clockwise starting at the dealer's left. The pot goes to the player with the highest valued five-card hand, with those five cards being selected from what's on the table and what's in hand (in the pocket).
Somebody should win off of "pocket aces."
Another thing to know is that's a bad idea to try to "fill the inside straight." This is where you would have a straight if you got one card to put in the middle of four cards you already have.
Sometimes people might try to buy the pot if they're bluffing, throwing in enough money that nobody else considers it worth meeting their bet.