You can ride with one module and replace it with another one at any tome, but what you can't do is ride with one module until it's run down, then inset another one or two with it. All the modules need to be the same voltage (normally fully charged) when you connect them together. You can take any one out at any time, but if you add another, it needs to be at the same voltage as whatever is already in there.
The problem is that lithium batteries go down in voltage as they discharge. If you connect two things at different voltages, the power will rush out of one into the other until the voltage is the same. That rush will blow a fuse or could make something catch fire. the bigger the voltage difference, the bigger the rush.
I can imagine that with one module, you'll get a lot of voltage sag and it would struggle for power, so probably better to run with at least two, or three if you're heavy.
We need someone to be a guinea pig to try one of these things and report back how it performs.