Fermented cucumbers without or with minimal salt possible?
Yes, you need salt. Salt selectively inhibits mold and bacteria which would otherwise out-compete the lactic acid bacteria. I'm sure you've seen vegetables rot; that's fermented cucumbers without salt.

There's no absolute minimum salt concentration. The less salt you add, the funkier and slimier the final product will be, and the more likely you'll be to end up with rotted vegetables. If you're using less salt, you'll also need to be more careful about fermentation temperature, and you may need to directly inoculate the pickles with a selected strain of lactic acid bacteria to help them compete.