Are the following items only used for crafting cosmetic things in the tack room?

Bones

elegant feather

White Feather

Plain Feather

Biogas bubble

Leather

Living Wood

Wing Membrane

What about Fire-forged Scale? Are there any others that if I destroy I won't be hampering my development?

Yes, they are used for crafting tack and possibly backgrounds. Fire-forged scale has the same use. Some of the gems are used for cosmetic but can fetch a decent price in the auction house. I think ruby is the only one needed for upgrades but I could be wrong.

If you go to help and list of recipes, it will show you what each building/upgrade requires.

Yes, you need rubies to build a ruby ring and a ruby pendant.

Believe me, those tack items are worth something, don't destroy them. They'll supplement your income.