Are paving bricks differently sized for sand bedding vs mortar bedding?
You can use the smaller brick with no stability problems.
The only issue you will have is if you chose a "basket weave" pattern when the brick are laid tight. The width of 2 bricks will not add up to the length of a brick side since these are sized with a mortar joint in mind.

 

If you use a stack bond, running bond or even herringbone, you can lay them tight with no issues at all.