Typically the game is played with 6 marbles/pegs for 3+ players, and 10 pegs for 2 players. However, if you are making your own variation that involves using more than 10, I suggest using 15 and filling the next row of the triangle.
Your answer is right in your comment. If he's flirting with your best friend and he has had three other girlfriends and told you he doesn't like you, then, minimally,...
Yes, you can set up multiple groups and direct messages with one account. GroupMe is great for cross-platform communication, for example from Android to iOS.