Today I disenchanted a fire resist item and tried to enchant Glass Armor with the resist using a filled Grand Soul Gem. It told me I could not use that enchantment for that piece. I was under the impression that resist enchantments were FOR armor pieces.

My enchantment skill is 35 in case that has anything to do with it. Any info would be great since I am really starting to get walloped by mages and need some resists. Thanks in advance.

Certain pieces can take certain enchants (ie. no fortify alchemy on a shield).

Hope the following helps - listing of what you can enchant with which enchants...
http://www.uesp.net/wiki/Skyrim:Enchanting_Effects

Ahhh, thank you very much.