I am in the camp of the 3.0 rules: No PC classes, NPC classes only.
By the way, Fantasy Flight Games has a workable "Thug" NPC class in their D20 supplement books; this makes a workable NPC substitute for Rogues (although Thugs are more leg-breakers than safe-crackers) to fill out a Thieves' Guild.
For religions, I created a Friar NPC class that is a Cleric-lite, with less spell or combat ability. Likewise, I worked out a Hedge Mage NPC class to be a Wizard-lite for the same purpose (although the Adept can work, as well; however, the mix of arcane and divine spells did not suit the theme I was seeking for my world).
Now.. assuming you decide to allow PC classes as followers... some additional comments.
Typically, Followers are the people who come to you to volunteer (as mentioned previously), so you don't get to pick them. One or more Cohort(s), who you do go out and recruit, would be the ones whose class(es) you get to pick.
The US Presidential election being topical, I will put it this way: Followers are like the people to volunteer to support a candidate's campaign; they show up, offer to help, and are put to work by the regular campaign staff. Cohorts, on the other hand, are like the candidate's advisors and campaign manager ... professionals chosen by the candidate for their skills. You could have a professional (lawyer, doctor, etc.) working as a volunteer or as a staffer, but generally the professional is not using his/her skills as a volunteer: the doctor is stuffing envelopes just like the young person from McDonalds.
Allowing the player to select the class(es) of his/her followers should make the process take more time. Instead of accepting just whoever shows up, the PC now has to spend time interviewing everyone who offers ... rejecting some and accepting others. That is likely to make the process take a good long time.
Hope that helps.