Right now just using 5 channels,
I would have thought that only having 5 channels should give you no very great latency, certainly not enough for you to see an appreciably difference when you implement the digital trigger so you only read the sensor being struck at the time.
If I can tie the digital pin to the output of the op-amp
Yes you can. Especially if the op amp is running off just the 5V rail.
where it looked like they had connected the pin to negative to signal low - is that how it's done?
You can. An op amp amplifies the difference in voltage between the + and the - inputs. You can put the signal into any one you like and fix the other at the threshold voltage with either fixed resistors or a pot. Without any feedback resistors the output is high if the + input is greater than the - input and low if it is the other way round.
Any thoughts on using a schmitt trigger?
In effect this is the same thing but with a fixed threshold voltage and a bit of hysteresis. You can add hysteresis to the system described above with a positive feedback resistor going to the fixed threshold voltage.
Look at the Comparators and detectors section of this link:-