Kinematics
Hero
Moving on the same round as an ally is actually a notable advantage. In particular, trying to coordinate actions between two PCs usually requires that one of them drops to the same initiative as the slower, just so they can make it work. That's because being able to both act together is frequently more powerful than each acting separately. After all, the enemy can't move, or hide, or heal, or anything else in between the two characters' actions, and that limitation is a painful hindrance.
So while it's understandable that you want to deal with all the NPCs as a batch, instead of rolling and tracking initiative for a dozen nameless goblins, letting them act together is like getting a free coordination bonus, permanently.
If I had a large number of NPCs to work though, instead of rolling one initiative value, I'd probably instead split their individual initiatives up evenly across the spread of 20. So if there were 10 goblins, one goes at 20, one at 18, one at 16, one at 14, etc. A large number of monsters should average an even spread, so if you want to simplify the results, it's better to spread all the monsters out, rather than make one roll for all the monsters. If they are more coordinated creatures (say, militaristic hobgoblins), maybe you can take pairs at a time, since they naturally work together, though still don't have every single one act at once.
So while it's understandable that you want to deal with all the NPCs as a batch, instead of rolling and tracking initiative for a dozen nameless goblins, letting them act together is like getting a free coordination bonus, permanently.
If I had a large number of NPCs to work though, instead of rolling one initiative value, I'd probably instead split their individual initiatives up evenly across the spread of 20. So if there were 10 goblins, one goes at 20, one at 18, one at 16, one at 14, etc. A large number of monsters should average an even spread, so if you want to simplify the results, it's better to spread all the monsters out, rather than make one roll for all the monsters. If they are more coordinated creatures (say, militaristic hobgoblins), maybe you can take pairs at a time, since they naturally work together, though still don't have every single one act at once.