An "unconditional" Damage bonus would also allow a character to combine fire, but a Mouse Droid can't get any Damage bonuses anywhere, let alone an unconditional one.
Most Damage boosts only kick in based on who or how the character is attacking. To allow a character to combine fire, it would have to be a Damage bonus that they have even while not attacking. For example, gaining Opportunist from Thrawn doesn't allow anyone to combine fire, but gaining +10 Damage from Talon Karrde would allow a character with 0 printed Damage to combine fire.
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