A single encounter involving 15 goblins all with different HPs sounds like a massive headache for the DM and players.

However, having a first combat with goblins all at 7HP, and then (X sessions or levels later) having another combat with goblins all at 10 or 12 HP, representing "elite goblins," is perfectly in line with the rules and an easy change the DM can make for more difficult encounters.