Here is a sample Ftr16, Ftr12/Mnk12, Ftr12/Wiz12, Mnk16, and Wiz16.
Regdar
NG Human Ftr16
Init +5; Senses Listen +12, Spot +12
Languages Common
AC 34, touch 14, flat-footed 33; Dodge, Mobility
hp 172
Fort +19, Ref +9, Will +11
Spd 20 ft.
Melee +5 bastard sword +27/+22/+17/+12 (1d10+16)
Ranged +5 composite longbow +21/+16/+11/+6 (1d8+12)
Base Atk +16; Grp +23
Atk Options Blind-Fight, Cleave, Combat Reflexes, Improved Bull Rush, Power Attack, Spring Attack
Abilities Str 25, Dex 13, Con 20, Int 10, Wis 12, Cha 8
Feats Alertness, Blind-Fight, Cleave, Dodge, Exotic Weapon Proficiency (bastard sword), Greater Weapon Focus (bastard sword), Greater Weapon Specialization (bastard sword), Improved Bull Rush, Improved Initiative, Iron Will, Mobility, Power Attack, Quick Draw, Spring Attack, Weapon Focus (bastard sword), Weapon Specialization (bastard sword)
Skills Intimidate +18, Listen +12, Spot +12
Possessions +5 full plate, +5 heavy steel shield, +5 bastard sword, +5 composite longbow (+7 Str), +6 belt of giant strength, +6 amulet of health, +3 ring of protection, +4 cloak of resistance
Martial Artist Regdar
NG Human Ftr12/Mnk12
Init +9; Senses Listen +24, Spot +24
Languages Common
AC 37, touch 27, flat-footed 37; Dodge, Mobility
hp 98
Immune normal disease, poison
Resist improved evasion
Fort +15, Ref +18, Will +20 (+22 against enchantments)
Spd 70 ft.
Melee unarmed strike +19/+19/+19/+14/+9 (2d6+5)
Ranged shuriken +19/+19/+19/+14/+9 (1d2+5)
Base Atk +12; Grp +17
Atk Options Blind-Fight, Cleave, Combat Reflexes, Improved Bull Rush, Improved Trip, ki strike (magic, lawful), Power Attack, Spring Attack, Stunning Fist 12/day (DC 23)
Special Actions abundant step 1/day, wholeness of body (24 hp/day)
Abilities Str 20, Dex 20, Con 14, Int 10, Wis 24, Cha 8
SQ slow fall 60 ft.
Feats Alertness, Blind-Fight, Cleave, Combat Reflexes, Dodge, Greater Weapon Focus (unarmed strike), Greater Weapon Specialization (unarmed strike), Improved Bull Rush, Improved Initiative, Improved Trip, Improved Unarmed Strike, Mobility, Power Attack, Spring Attack, Stunning Fist, Weapon Focus (unarmed strike), Weapon Specialization (unarmed strike)
Skills Balance +22, Climb +17, Escape Artist +20, Jump +35, Listen +24, Spot +24, Swim +17, Tumble +22
Possessions +8 bracers of armor, +6 belt of giant strength, +6 periapt of wisdom, +6 gloves of Dexterity, +5 ring of protection, +5 cloak of resistance, pink rhomboid ioun stone
Arcane Regdar
NG Human Ftr12/Wiz12
Init +8; Senses Listen +8, Spot +8
Languages Common, Draconic, Elven
AC 25, touch 17, flat-footed 21; Dodge, Mobility
hp 113
Fort +16, Ref +13, Will +15
Spd 30 ft.
Melee +3 greatsword +23/+18/+13 (2d6+13)
Ranged +2 composite longbow +18/+13/+8 (1d8+8)
Base Atk +12; Grp +18
Atk Options Blind-Fight, Cleave, Improved Bull Rush, Power Attack, Spring Attack
Spells Prepared
6th-3
5th-4
4th-4
3rd-5
2nd-6
1st-6
0-4
Abilities Str 22, Dex 18, Con 16, Int 22, Wis 10, Cha 8
SQ toad familiar
Feats Alertness (if familiar is within 5 ft.), Blind-Fight, Cleave, Dodge, Empower Spell, Greater Weapon Focus (greatsword), Greater Weapon Specialization (greatsword), Improved Bull Rush, Improved Initiative, Iron Will, Mobility, Power Attack, Quicken Spell, Scribe Scroll, Spring Attack, Weapon Focus (greatsword), Weapon Specialization (greatsword)
Skills Concentration +18, Decipher Script +21, Intimidate +11, Knowledge (arcana) +21, Knowledge (the planes) +21, Knowledge (religion) +15, Listen +8, Spellcraft +23, Spot +8
Possessions +8 bracers of armor, +3 greatsword, +2 composite longbow (+6 Str), +6 belt of giant strength, +6 headband of intellect, +4 gloves of Dexterity, +4 amulet of health, +4 ring of protection, +5 cloak of resistance
Ember
LG Human Mnk16
Init +9; Senses Listen +26, Spot +26
Languages Common
AC 37, touch 29, flat-footed 37; Dodge, Mobility
hp 123
Immune normal disease, poison
Resist improved evasion; SR 26
Fort +18, Ref +20, Will +22 (+24 against enchantments)
Spd 80 ft.
Melee unarmed strike +18/+18/+18/+13/+8 (2d8+2)
Ranged shuriken +18/+18/+18/+13/+8 (1d2+2)
Base Atk +12; Grp +13
Atk Options Combat Reflexes, Improved Trip, ki strike (magic, lawful, adamantine), quivering palm (DC 25), Stunning Fist 16/day (DC 25)
Special Actions abundant step 1/day, wholeness of body (32 hp/day)
Abilities Str 14, Dex 20, Con 16, Int 10, Wis 24, Cha 8
SQ slow fall 80 ft.
Feats Blind-Fight, Combat Reflexes, Dodge, Mobility, Improved Initiative, Improved Trip, Improved Unarmed Strike, Spring Attack, Stunning Fist, Weapon Finesse, Weapon Focus (unarmed strike)
Skills Balance +24, Jump +42, Listen +26, Spot +26, Tumble +26
Possessions +8 bracers of armor, +6 periapt of wisdom, +6 gloves of Dexterity, gauntlets of ogre power, monk's belt, +5 ring of protection, +5 cloak of resistance, pink rhomboid ioun stone, dusty rose ioun stone, pearly white spindle ioun stone
Naull
NG Human Wiz16
Init +5; Senses Listen +11, Spot +11
Languages Common, Draconic, Elven
AC 28, touch 20, flat-footed 23
hp 121
Fort +15, Ref +15, Will +16
Spd 30 ft.
Melee +1 quarterstaff +8/+3 (1d6)
Base Atk +8; Grp +7
Spells Prepared
8th-2
7th-4
6th-4
5th-5
4th-5
3rd-6
2nd-6
1st-6
0-4
Abilities Str 8, Dex 20, Con 20, Int 24, Wis 12, Cha 10
SQ pseudodragon familiar
Feats Alertness (if familiar is within 5 ft.), Empower Spell, Greater Spell Focus (conjuration), Greater Spell Focus (evocation), Greater Spell Penetration, Improved Familiar, Maximize Spell, Quicken Spell, Scribe Scroll, Spell Penetration, Spell Focus (conjuration), Spell Focus (evocation)
Skills Concentration +24, Knowledge (arcana) +26, Knowledge (the planes) +20, Knowledge (religion) +12, Listen +11, Spellcraft +28, Spot +11
Possessions +8 bracers of armor, +1 quarterstaff, +6 amulet of health, +6 headband of intellect, +5 gloves of Dexterity, +5 ring of protection, +5 cloak of resistance
************************
I don't see a huge disparity in power here. The multiclass characters are not vastly superior in any way since they have to split their disciplines. Nor is the fighter/monk inherently worse than the fighter/wizard. The fighter/monk has fewer defensive abilities but very similar AC and attack bonus to the monk. The only downside is hp, but improved evasion has to account for that somewhere, or the fighter/monk could get an amulet of health +6 instead of a periapt of wisdom +6 (lowers his AC, Will save, Spot and Listen, and stunning fist save DC by 3 though). Makes me think maybe you just let them roll hp every 3 levels for multiclass levels instead, since that would give the fighter/monk more hp.