When building the ramps 1.4 is it safe to use small wires to jump the 5A, 12V power input over to the 11 Amp input
I guess that the external split is at least partly done to force the user to utilize 4 wires. 16 Amps at 12 Volts is quite a bit of power and you don't want your wires or connectors to melt. If you use twice the wire cross section by using 4 instead of 2 wires, you also reduce the resistivity and hence heat created in the wire.

Put it the other way around: Using 2 instead of 4 wires raises the risk of your setup to catch fire.

Still, you can use that bridging technique, if you make sure that all your wire diameters are big enough.

I would advise you, however, to cut the wires that come with your kit in half and use the connectors in the way they were meant to. For optical appeal and less cable clutter you can still put the wires into some braided sleeve for example.