"Some old Li-Ion cells" are the worst things to make a battery of, no matter if it is parallel, serial or mixed.
They will differ in both capacity and state-of-charge vs voltage curves, so no BMS will make them play together for long.
You can, of course, spend some time with a battery analyzer and match some of them for series connections. If they are of the same brand and model, you may as well connect them in parallel.
Good luck if you expect all of them to age at similar rate.
I am not saying it is not going to work, it is simply a waste of time and resources.
As far as I remember, Mac Mini is like ~30W max. Not that it is impossible, but it is still quite unusual combination of parameters that will hardly be efficient.
And you will hardly find 30W 3.6V input DC-DC converter off-the-shelf, you will have to make it yourself.
Now wait, you will want a monitor for your mac mini as well, ... ? 20-50W more to go.
The scooter - 100-300W at 3.6 volt is quite a challenge. The connecting wires will probably heavier than both the batteries and the dc-dc together.
A battery shorting in your setup is your least concern. The usual failure mode for Li-Ions is to fail open and not short. Most factory-made battery packs that feature parallel connections are simply welded together.
A thermal fuses are probably a better option than glass ones. A calibrated nickel stripe for welding them together is even better.