Which plural do you use for a word that should have a regular plural but ends with a word that has its own irregular one? The example that made me ask was "leatherman" (the multitool) but there are plenty of others.
Short answer: it's up to the company, but nobody cares what the company says.
The official plural of Walkman according to Sony is Walkman Personal Stereos, not *Walkmans or *Walkmen. People widely use walkmans nonetheless.
Since Leatherman refers to a brand name and not a specific product, there probably is no official plural—even though Leatherman is used quite often colloquially to refer to a Leatherman-brand multitool. But people say Leathermans nonetheless. Seeing a pattern?
One more amusing example: Bluetooth is used colloquially on its own to refer to a Bluetooth phone headset; a cursory Googling shows Bluetooths dominated by Bluetooth-related results, while Blueteeth gives mixed results, including a number of shoddily edited product sites, one of which claimed that the “pecifications” of a phone include “blueteeth”.
So that's something.
The general rule is- If the compound word is a type of the base noun, pluralize as the irregular. Otherwise pluralize the compoud as a regular noun. Fireman becomes firemen Walkman would become Walkmans Leatherman would become leathermans. Of course trademark names are not subject to the usual rules.
In this case the tool is named after a Mr Leatherman - so it's a 'leatherman tool' (sing) or 'leatherman tools' (plural).