Can a TTL LS device (low-power Schottky) serve as an input to a basic TTL device?
For the circuit as shown, yes.  According to my late-70's databook, 74LS parts have an output current of 400\$\mu\$A at 2.5V out, while 7400-series logic has an input current of 40\$\mu\$A (the similar rating for 74LS logic is 20\$\mu\$A).  You certainly can't drive as many 74xx inputs from a 74LSxx output as you can from a 74xx output, but just as certainly you can drive one.

I was going to check the fan-out ratings of 74LSxx logic, but as I was turning to the paper book I realized that it didn't have a search bar, and desisted.