For myself, I would not allow that progression with levels in different classes. From my perspective, the classes are not something you can just take. The package of abilities represent years of training for a starting character- even a fighter learning to utilize all those weapons proficiently takes a significant amount of time.. Once adventuring begins, there would need to be significant amount of downtime so characters can't be wandering off adventuring (I use the training variant in the DMG, but the times are longer).
My own preference as a DM for those types of backgrounds would using be, if using 3e a combination of UA style class variants (discussed briefly in the PHB and DMG), 3.0 0/0 multiclassing at first level, and/or a third party class to help get close to the concept at first level.
For the ex thief, I would tell the player to
a) use the Martial Rogue UA variant ( a rogue that gives up sneak attack for bonus feats) and Paladin multiclass at first level;
b) I might create a Paladin variant for the thieving while the fighter training would be covered under Paladin training and weapons- use your starting feat and take a feat on the fighters list; or
c) some combination of the above
This would cover the character up to the point the adventure begins. Any probation period would be factored in during the paladin training. At most, the character would have two classes at the start and either way the background elements are covered up to the start of play.
Depending upon the campaign setting, I might ease the training restriction on multiclassing to cleric since the Paladin is a divine class. Then again, I might not if the campaign setting there are rituals and other things that need to be learned to become a "priest" while Paladin is simply a reward for faith or calling from deity in which case the Paladin goes out of game for a period and can come back later.
For your mystic, I would, probably, tell the player to take the OA Shaman. The class can fight unarmed and gets spells. If necessary, I would talk with the player and create a tailored spell list variant in the DMG would be employed. It is all done with one class.
Again, just my own take for when I run.
My own preference as a DM for those types of backgrounds would using be, if using 3e a combination of UA style class variants (discussed briefly in the PHB and DMG), 3.0 0/0 multiclassing at first level, and/or a third party class to help get close to the concept at first level.
For the ex thief, I would tell the player to
a) use the Martial Rogue UA variant ( a rogue that gives up sneak attack for bonus feats) and Paladin multiclass at first level;
b) I might create a Paladin variant for the thieving while the fighter training would be covered under Paladin training and weapons- use your starting feat and take a feat on the fighters list; or
c) some combination of the above
This would cover the character up to the point the adventure begins. Any probation period would be factored in during the paladin training. At most, the character would have two classes at the start and either way the background elements are covered up to the start of play.
Depending upon the campaign setting, I might ease the training restriction on multiclassing to cleric since the Paladin is a divine class. Then again, I might not if the campaign setting there are rituals and other things that need to be learned to become a "priest" while Paladin is simply a reward for faith or calling from deity in which case the Paladin goes out of game for a period and can come back later.
For your mystic, I would, probably, tell the player to take the OA Shaman. The class can fight unarmed and gets spells. If necessary, I would talk with the player and create a tailored spell list variant in the DMG would be employed. It is all done with one class.
Again, just my own take for when I run.
Part of my approach is my view to mechanics. To me, a level is not a definite piece of a character - it is just a package of abilities for the character. You get feats, you get ability score increases, you get levels.
Think of this character (a concept I've played). Son of a Noble in a line of Paladins - but he is out with his parents on a trip (he's about 8). They all get killed, and he barely escapes - and tramautized ends up in a city as a classic dickensian street thief. Then someone recognises him, and brings him to the temple - with his background he belonds in the temple to be a paladin, but because of his actions, he needs probation. So he is a fighter for a while, then he wins probation, and becomes a paladin. After gaining paladin spells, he realizes his true calling is in serving his god as a priest.
The character story arc is pretty standard (redeemed thief) - all of the choices are made in character, not for power gaming, or because of plot reasons (the probationary period) - it is completely organic with the character and his experiences. Say he spends 2 levels in each class until Cleric - that makes him a 7th level character as a first level cleric.
Why should someone with that level of character growth, who's beliefs and backgrounds should be penalized because the course of the character happens to fall across 4 classes.
Perhaps you have a character idea of a "Mysitc" - start of with physical training (Monk) and after mastering physical abilities (say monk at 5th or 6th) you move into even more mystical realms (and become a Sorcerer or Psion) of the "mytic path." That is only two, but very basic - the character is not a Monk, nor a Sorcerer or Psion - he is a "Mystic" as a character type - one which the player models by having multiple classes.
The first case uses classes as normally seen - an order or oginization for each class as a social role. The second, class levels are no more than a tool used to create the mystic.
I don't see why that would be bad.
Especially with "each class has it's own thing that no one can do better" - being able to freely multiclass keeps that from being a straightjacket -
Another thing I'd want from 5th - no formal roles stated and adventures set up that you don't have to cover all roles (fighter, cleric, thief, wizard) to complete - all fighter and all wizard parties are just as effective (if different) than a mixed group.
