Don't let the fact that the clerk at the home center appeared perplexed. They are seldom the "experts" that they proport to be.
Yes, you are correct that the neutrals and the grounds need to be kept seperate in the subpanel. In some panels, that come factory equipped with two bars, you can seperate one bar from the other and use one for neutrals and one for grounds. The neutral bar must be insulated from the "can" of the panel, and the ground bar must be connected to the can. Still some panels contain only one bar, so you have to buy a "ground bar kit" (Such as Square D, 'QO'). Some panels come with two bars, but you still have to add a ground bar kit (such as Square D 'Homeline').
You can repost with the manufacturer and model of the panel that you propose to use, and an expert will respond with exactly how your panel is utilized as a subpanel.
"Nothing is foolproof to a sufficiently talented fool"