I wouldn't get the dwarf lion. Depending on the species of lion and angler, the angler is one of the few natural predators of the smaller lions. If the angler ever does try to eat it, they could both end up dying. Boxfish can get pretty big, and if they don't, they'd be a quick meal.
Often species get different sizes. The biggest thing is to keep them so they're always the same size, and watch what species you get. Thus, they need to start out the same, have the same growth rate, and same max size. You don't want a Commerson's, for instance.