re: Compatible Woods in Small Boxes

Picture does not refer to granadillo.
picture came from this site: http://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/2_Wood_Movement/2_Wood_Movement.htm

The 1/4 per 12" tolerance in the article probably refer to a wood that move most; I am thinking pine or the likes.

I think you have to worry of the bottom and the top only.

longitudinal and radial, I usually ignore.

Tangential is the beast. That’s usually where the width of the stock will be substantial. again with your stock dimensions, it is still a small movement.
Quatersawn, if I remember correctly, alleviate the wood movement.

in your case ash or maple movement would be very similar.

I am blessed to be in the San Francisco bay area where we do not have too much swing in humidity. Texas would have some more variation. The only time were I sweat for wood movement was for my outdoor picnic table.

Rabbeted or not the wood is going to move. Ash/maple and granadillo move at different rate.

Laminated strips of contrasting color could add some nice element to the design. Sketchup or photoshop couldbe useful to determine the final look. I personaly use the color printer combo I have. Cut and glue:)
I would just add a piece of ash next to my stock and see how it looks. you first picture on the red mat made me think padauck might work. Maybe too intense..

As for design and creativity, I am on the same camp as you.
I usually ask my wife’s opinion. Logic and design together is rare. I say let you idea flow. The wood you are starting with is beautiful.
The bottom is not visible from outside so I don’t care so much for it. the inside is usually lined with fabric/padding.
My boxes bottom has always been plywood.