Hello,
You can use multiple payment gateways and you can also make only specific payment method to be allowed for specific type of product (i.e. standard product or donation "product" type). There may be a need for some little coding in the payment method selector (in one of the checkout process step control) to offer only one of the methods, but it might be also achieved by proper Shipping methods setup (with proper payment options assigned to them). The particular solution may depend on other information, like if there should be some combined orders allowed, etc.
As for the Authorize.NET config, your assumption is correct. It's also described
in our E-commerce Guide (v.6)Should you need any additional details, please feel free to ask.
Regards,
Zdenek