With reference to typography I have the following questions:
1. Can Arial and Garamond fonts be mixed in a logo and what image do they create?
2. What script fonts are compatible for mixing with Arial and Helvetica in a logo. For example Mistral, Bickham Script.
Arial and Garamond can blend nicely as one is a serif font (Garamond) and the other is sans serif (Arial). I don't know exactly how you plan to use them, but generally, using one font for a main word and another for a "sub-title" works nicely. For example, for a company named "Johnson Automotive" you might consider putting "Johnson" in large, bold Garamond type and "AUTOMOTIVE" underneath in a smaller size in the Helvetica typeface with some decent letterspacing between the letters.
I would not recommend mixing multiple sans serif fonts (like Arial and Helvetica) within the same logo, or even the same print piece (like a brochure).
As for script fonts...they are so numerous and I use them so rarely, I really can't comment. I generally stay away from them unless it REALLY fits the vibe of the company the work is for. And when I do use a script font, my general concern is picking the most legible script font I can find.