Looking more at the Commerce Extra Rules Conditions module more, it looks like it can handle combined quantities based on both taxonomy and product type (I thought it was limited to Taxonomy at first), but it still doesn't handle defining specific SKUs, which may or may not be necessary as I get more into all the specific discounts, and the module seems to be abandoned for almost a year now in dev.

So, suggestions still greatly appreciated, and if anyone wants to take on maintaining that module, it seems like it would be a good benefit to the community.

I tried out the Commerce Rules Conditions module, which provided a fairly easy setup, but didn't allow for mix and match by SKU, and hasn't been maintained for some time. So I set up a discount template using rules components. I'll post the rules exports and some description below in case anyone wants to play with it, is also looking for a core rules-based solution, or needs something more flexible.

Rules exports zip at google code: http://code.google.com/p/drupal-commerce-mix-and-match/downloads/list

The main product pricing rule is fairly simple, executing on event "price is calculated," (so, on each line item in the cart) just setting some basic conditions (line item is product, with a discounts taxonomy field), then declaring variables (criteria, quantity and price calculation) and calling the main rules component. Conditions for SKU/Type/Term can be added to speed up rules processing time but it's the variables that create the discount criteria. This leaves the setup of each discount to be fairly simple.

From there, I set up 7 rules components:

1) Action Set Component to define variables and provide some default values, called from the main pricing rule as the first action. Criteria variables are list variables to allow for multiple criteria of each type.

2) Action Set Component which loops on each cart item to generate a list of matches, called from the main pricing rule as the last action. This means while calculating the price for each item, the rule checks for matches on all other items for each of the criteria types, generates the list of matches, then loops on the list of matches to total the quantity of matching items.

3) Rule Component to check for SKU-based matches, which sets a SKU match truth value, called from the cart line item loop.

4) Rule Component to check for Type-based matches, which sets a type match truth value, called from the cart line item loop.

5) Rule Component to check for Term-based matches, which adds a term match truth value to a list of truth values, called from a loop on defined term criteria within the cart line item loop.

6) Rule Component to add line item to list of matches if it meets any of the defined criteria. Handling this separately from the criteria checks allows for the option of combining term and type criteria to be more selective, or being more inclusive using term OR type criteria (which is set using a truth value variable in the initial pricing rule to combine type/term criteria).

7) Rule Component to evaluate total quantity of matches, check if current line item is in the match list and set line item price to the calculated price change set in the initial pricing rule.

Additional notes: The variable "Combine Type/Term" sets whether type and term criteria should be combined to be more exclusive, or not combined to be more inclusive. If the max quantity variable is set to 0, max quantity will be ignored. Any of the criteria can be left undefined. Obviously, for term criteria to work using the exports, you need to edit the rule and components to use your desired vocabulary (one condition in the initial pricing rule, and three conditions in the Term Matches rule component (#5)).