When creating items at a forge, all that matters is whether one has the perk that allows one to create an item using that particular material - e.g. to create ebony weapons & armour, you need to have the ebony smithing perk?
Also, items & potions that fortify smithing are of no use at item creation - their effects apply only when improving weapons and armour?

You do not need the perk to improve items(pretty sure you need it to create them though). Fortify smithing potions have been completely useless while improving items for me, completely worthless.

I believe there is some sort of improvement bonus associated with having the perk related to the item you are honing (e.g. if you improve an ebony weapon you will have better results if you have the ebony smith perk).

you get access to legendary if you get the perk which is just more damage.

For the potions to have any effect it has to boost you up enough to take you to the next level of smithing numerically. I have been able to ungrade a weapon to legendary on a weapon class that I didn't have the perk for by having 100 smithing, wearing smithing gear, and taking a high level smithing potion. But if you don't have the numbers to push you up to that level you won't see any improvement if that makes sense.

your level in smithing count too, level 100 vs level 50, will produce different result when fortifying the stuffs.