I have built 15 DIY force sensors: copper fabric/velostat/copper fabric sandwich, one side to 5v, the other to pin via pulldown 220R. If I connect them all in series, ie 15 x 220R (common ground) running off a single 5v pin, should I expect the same response off each sensor?
No, because you will now have 15 times the resistance in the circuit. The 220 Ohm resistor will now be much too low for the circuit to work.
Or will the current vary (reduce) from sensor 1 to sensor 15.
Current will be the same in all the sensors, because they're in series.
Should I use 15 digital pins (OUTPUT, HIGH) to power each sensor with its own 5v?
No, just use the 5V power supply pin(s). Probably best to power each sensor with 5V separately and use 15 separate 220 Ohm resistors, and then you'll have 15 analog signals but only six analog inputs.
Look up analog multiplexing for a way to handle that.