Using the SCARA motors, which have a 5.54A theoretical current draw at stall?
That's the real kicker right there. A motor shield is really nothing more than an H-bridge and associated support components. If you are handy with perfboard, you could build one yourself. I tend to use TA7291S (in a freindyl SIP package), but these are rated at 1.2A max. Putting two in parallel would handle 2.4A, and so on (starting to get pricey though.)
I would look for H-bridge ICs that can handle up to 6A. I have no idea how easily you would find one. The other option is to build 5 h-bridge circuits that can handle that load by hand or find a battlebot style board.
I am suggesting this because I am not aware of an existing shield that meets your specifications. If you do find one that drive that current, but only has a few outputs, you can usually stack shields.
Personally, I would just build my own. YMMV