Many RPGs use individual initiative and yet a common piece of DM advice to speed this up, simplify note taking, and get to the action is to roll a single initiative for groups of monsters. Effectively, the PCs get individual initiative but the monsters act as a group. This is certainly easier to track and large groups of monsters acting simultaneously can be terrifying. But due to the variance of the d20, the side with more chances to roll is more likely to have someone on their side go first. It's like dog-piling a skill check - someone is bound to succeed and the initiative bonus relative to the range of outcomes on a d20 means it's only slightly more likely the dexterous characters win.
So what's my point? Initiative should either be grouped or individual for all combatants.
Group Initiative
If you're going to do group initiative for monsters, do group initiative for the PCs. It's only fair. Unless your game leans strongly heroic, monsters ought to go first as often as PCs do, or at least more often than they do in the games I've witnessed. The PCs are always the aggressors, always the ambushers. They take tedious precautions to make it so and for what? A single surprise round or temporary advantage? If you want your game a little tougher, and your monsters a little scarier, give your monsters the chance to act first or actually execute an effective ambush.
This also keeps the players engaged. Everyone is invested in the group's actions and the opportunity to coordinate suddenly appears. No more clogged hallways or stairs due to initiative randomizing the action order of the stacked up SWAT team. Group initiative also allows the possibility of re-rolling each round (and thus the PCs or monsters acting twice in succession) without introducing excessive overhead - a great way to create additional tension and chaos from round to round.
Individual Initiative
If you go with individual initiative and there are multiple monsters, especially if they outnumber the PCs, roll for them as individuals or at least multiple groups even if all are of one type. Give them a fair number of initiative rolls and the subsequent opportunity to have their side act first. If nothing else, consider giving groups of monsters or "swarm" types advantage on their single initiative roll - their numbers should count for something here. Monsters having individual initiative adds chaotic and tactical implications to the combat and equal opportunity for advantageous outcomes.
The simple solution is group initiative. The complex one is individual for all - let's just make it fair and give the monsters their due.
Coming soon: Re-rolling Individual Initiative, the Best of Both Worlds?
