Gotcha. So, I have two concepts that would address this by a template modification to accomplish either of the below, or possibly a functions file modification (or of course a combination of the two).
There is a template modification that *is* in this forum but may also be in the instructions (been a spell since I've looked at that aspect) that will make the quantity box effectively 1 without allowing the customer to add multiple quantity of groups from the product information page, but would allow modification on the shopping cart. A next modification would be to allow adding multiple groupings but start with a quantity of 1 instead of the unit/min mix that ZC offers its standard product.
The function modification (a core file edit) would be to return a value of 1 if the product in question has a grid attribute instead of this unit quantity.
A combination style would be to use a new function that is called by a modified template file when there is a unit/min related requirement such that the option could maybe be controlled by some database setting...
As to checkboxes... unfortunately generally speaking they are problematic and best used for a two state situation such as giftwrap or not. It can be factored in as a "quantity" style product, but that requires additional inspection upon submission and handling to identify the errant condition. Not saying it's hard, just more involved.
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