I wouldn't recommend that. A fifth level fighter/rogue (2/3 maybe) with magical support? He should eat the paladin for breakfast and quite possibly the rest of the party as well. This is where the problem that others and I mentioned comes up: appropriate challenges for their level are weak because the party has much more offense but harder challenges will tend to eat them for breakfast because, despite level 2-3 quality offenses, their hit points are still level 1 and their ACs are probably no higher than level 2.
So take a sample 32 point Ftr 2/Rog 3
HP 38; Init +7, AC 20 (24 with haste, 27 with shield, 31 with haste and shield); Atk +9 melee (1d8+3 longsword) or +10 ranged (1d8+3 mty +2 comp longbow); SA Sneak Attack +2d6; SQ: Evasion, Uncanny Dodge; AL CE; SV Fort +7, Ref +7, Will +5; Str 16, Dex 16, Con 14, Int 10, Wis 12, Cha 10
Skills and feats: Bluff +8, Climb +1, Decipher Script +5, Disable Device +6, Hide +9, Jump +8, Move Silently +9, Open Lock +7, Sense Motive +6, Tumble +11, Use Magic Device +10; Improved Initiative, Iron Will, Weapon Focus: Longsword, Power Attack, Cleave.
Equipment: +1 chain shirt with armor spikes, +1 ring of protection, masterwork buckler, +1 cloak of resistance, Masterwork longsword, masterwork dagger, Masterwork Mighty [+3] Composite Longbow, 20 masterwork arrows, Potion of Cure moderate wounds, 2 potions of cure light wounds, potion of invisibility, potion of haste, 4 scrolls of Shield (clvl 1)
In the first round, he's likely to win initiative and drop the party's wizard to -6 (average damage: 14 assuming the wizard has 18 con) with a sneak attack (because he's nervous about color spray, etc) and either cleave into the paladin or someone else (and he'll drop the paladin too on average damage--assuming anything less than an 18 con for the paladin) if in melee or prepare to flank with his magical flunky. If he's feeling really threatened, he might open up by drinking his potion of haste and reading his scroll of shield (50% chance of success but if he fails, he's got several others and getting the shield up in the next round) which would give him an AC of 30--even the paladin would need to roll 20's to hit him. After that, since he's immune to Command and Sleep (5 HD) and the worst color spray could do is stun him (which would be inconvenient but not the end of the world, he'd probably walk over the rest of the party too. Even assuming the average member of the party has AC 20, he'd have a very good chance of dropping 2 party members a round with cleave. And that's without including any flunkies or support other than the standard wealth for a 5th level NPC.
Were the party 2nd or 3rd level, they'd stand a fairly good chance of prevailing since it would take the villain two attacks to drop any PC (except a wizard) and at 3rd level, the PCs would get access to things like Sound Burst, Hold Person, Glitterdust, and Tasha's Hideous Laughter which could exploit his weak will save and make him much more susceptible to attack or Web which would probably slow him down long enough for the haste to wear off.
Xarlen said:
Well, a paladin is immune to Mummy rot and Fear, so... 
No, I don't plan on putting them against That. Though, at the moment, I can't think of a Lot of thing to challange them with (Maybe bandits? Not sure). But I've got ideas. However, the BBEG is still allowable (I'm thinking a 5th level fighter/rogue, around there, with magical output; With the paladin swinging at +6, not a bad choice).