Some diveshops have a regrettable tendency especially in low season to try to sell you whatever they have in stock rather than what you really need.
AL makes an octopus specifically for the Legend which is balanced so as to avoid any problems with the way the Legend increases IP at depth. Other AL octopi have a venturi switch to desensitize the octopus which can serve somewhat the same purpose. Most of the Sherwood octopi, best I can recall, have neither. It's hard to say how necessary this is in real life, but it makes sense to me to use the octopus AL recommends since one is available.
It also makes sense from a service and resale point of view to stick with the same-make octopus. If you had the Sherwood kicking around, I'd say try it, or if they were giving you a hell of a deal on the Sherwood and would take it back if it didn't work out (though you won't know for sure if it does until to get down deep), but if you are paying full price for an octopus, why not get the right one and be sure it will work?