Just out of curiousity, how would a mouse be any different from a rat, statistics and ability wise?
Not much. Here's the DM guide mouse:
Mouse: No familiar is as small and innocuous as a simple mouse, able to move almost anywhere unseen. Mouse familiars provide their masters with a +2 bonus to Hide checks. Mice cannot do any damage in combat. They eat vegetables and grains, primarily.
Mouse: CR -; Fine Magical Beast; HD 1; hp 1/2 master's; Init +0; Spd 10 ft., climb 10 ft.; AC 19 (touch 18, flat-footed 17); Base Atk +0; Grp -21; Atk -; Full Atk -; Space/Reach: 1/2 ft./0 ft.; SQ improved evasion, scent, granted abilities; AL Any; SV Fort +2, Ref +2, Will +1; Str 1, Dex 11, Con 10, Int 6, Wis 12, Cha 2.
Skills and Feats: Balance +8, Climb +10, Hide +20, Move Silently +12.
And here's a dire mouse we came up with for a different conversion...
Dire Mouse
Small Animal
Hit Dice: 1d8 (4 hp)
Initiative: +3
Speed: 40 ft. (8 squares), climb 20 ft.
Armor Class: 15 (+1 size, +3 Dex, +1 natural), touch 14, flat-footed 12
Base Attack/Grapple: +0/–6
Attack: Bite +4 melee (x-2)
Full Attack: Bite +4 melee (x-2)
Space/Reach: 5 ft./5 ft.
Special Attacks: -
Special Qualities: Low-light vision, scent
Saves: Fort +3, Ref +5, Will +3
Abilities: Str 6, Dex 17, Con 10, Int 1, Wis 12, Cha 2
Skills: Balance +11, Climb +11, Hide +17, Jump +15, Move Silently +13, Swim +11
Feats: Stealthy, Weapon Finesse (B)
Environment: Any
Organization: Solitary or family (5–20)
Challenge Rating: 1/4
Treasure: None
Alignment: Always neutral
Advancement: 2–3 HD (Small)
Level Adjustment: -
Skills: Dire mice have a +4 racial bonus on Hide checks and a +8 racial bonus on Balance, Climb, Jump, Move Silently, and Swim checks. They can always choose to take 10 on Climb checks, even if rushed or threatened. Dire mice use their Dexterity modifier for Climb, Jump, and Swim checks.