This is the short-winded version of our post regarding our findings on character generation for a pacifist run through Baldur's Gate I:
Both groups of our players who tried a pacifist run through Baldur's Gate found the combination of an Enchanter + Thief to be the best solution for the vast majority of the game.
The Thief's ability to level up reasonably quickly and build up their stealth skill will enable them to intermittently hide in the shadows (becoming invisible to enemies), and Mages can also eventually learn to cast invisibility spells that last even longer, but with a limited number of castings before rest in an enemy-free area is required.
When both a Thief and Mage capable of casting "Charm" (an Enchantment spell) are operating together, many quests that require you to get a key quest item from a hostile enemy can still be solved by charming the enemy and pick-pocketing them while charmed, so you can still solve the quest. (Otherwise, the game unfortunately makes it impossible to pickpocket hostile creatures even if they're unconscious or incapacitated, such as through "Sleep" or "Color Spray" spells.)
The builds above allowed us to sneak through most of the game, gaining nonviolent rewards of quest completion experience points that was scarce, but still plentiful enough to allow our mages and thieves to achieve high enough levels to cast good spells and use effective stealth skills.
Depending on what version of the game you have, it might be possible to play a Chaotic Good Mage and select the "Find Familiar" spell at level 1 to get a Faery Dragon familiar. (Our version came with the Faery Dragon's ability to cast a high-level invisibility spell that could be cast on a whole group of allies at once, but we saw a few remarks online indicating that this ability might've been taken away with later updates of the game.) Using a familiar is optional, however, regardless of what abilities they have in your version: Our run with a halfling rogue/thief as player character did not use one at all.