Lord Vangarel said:
What solutions could I use to prevent future problems?
I could create the cohorts. This would stop a player using it as an extra ability to powergame.
I could roleplay the cohorts. This would stop any additional time being taken by a player with a cohort reducing other players time.
Both of these are good ideas, although you don't need to be so absolute on it. For the creation of the cohort, you can work with the player to pick something appropriate. If he's affiliated with a church, his cohort probably should also (i.e., the Cleric gets a Paladin bodyguard or vice versa). If he's a spellcaster of one type of magic, his cohort could the other class that uses the same type of magic. And so on. In my case, my Aristocrat/Psion (yes, I took an NPC class level) wanted an apprentice, so the DM and I designed a Psychic Warrior to be his student. We tweaked her stats as we went along, retroactively changing out old levels if it made more sense for the character.
As for roleplaying the cohort, it's still important to let the player handle some of the day-to-day stuff, like picking which spells to memorize, or running the cohort in combat. If the cohort's the "strong 'n stupid" type it's not a big deal, but for casters it's really important for the character to not be played by someone who knows all the secrets of the encounter.
Actually, back in high school I was in a group that had a great solution. There were almost a dozen players, which got unmanageable at times, so one guy (who couldn't make it every week and so didn't want to make a normal character) volunteered to be NPC Guy. He was sort of a DM's Assistant who roleplayed all the minor characters.
Anyone couldn't make it that week? NPC Guy ran their characters for them.
Cohorts, Followers, Familiars, Mounts? Same. (We were all level 12 or so, so there were a lot of these). If the DM had an encounter where he needed a second personality, he'd hand the stats of some of the "thug" bad guys to NPC Guy (but not the masterminds who actually had to know what was going on.)