Well, I'd have to write a routine to convert one format to another. Then all shop code can use the same logic.
I could definitely squeeze out enough space if I can move stuff around -- but to make this "patch friendly" I can't really move stuff around. =/
I think it's definitely important to make it as "patch friendly" as possible... In the original FF1 it's already hard to know which weapons can be equipped by who and the only real way to know without wasting money is to look it up. Now hacks often changes weapons, which means it's even harder to "guess" who can equip what. With harmour it's the same nightmare. If you mess up and buy something you didn't need, you're effectively wasting half of the item's price as it sells back for half price.
For magic it's less hard to know who can equip what, because often there's some kind of logic behind this, black mages can learn all black magic and white mages all white magic, and red mages everything but up to a certain level. The only issues is for upgraded classes which can equip some low level magic, but this is rarely useful as those are obsolete by the point you can upgrade your party (healing spells don't heal enough, offensive spells don't hurt enough, so basically this is a worthless and gimmicky feature). For hacks with new classes, such as this very FF Redux, it might be very useful though, because we don't know how the hacks author mapped new spells to new classes.
If such a hack is not possible, then at least make is so that items are sold for the same price as they're bought. That way we can buy an item, test, and if it's useless we can just return it without wasting money.