Playing Lunia, having the Ultimate skill on your cash item will not give you the Ultimate Skill unless you have read the Ultimate Skill book previously. You can apply Dyes to all cash items, including level 1 cash items or ones you get from Star Boxes.

Also that is possible to get +2 to the same skill if you use 2 of the same skill dye on the same cash item. It's possible to add a skill using the skill dye to a cash item which original has a skill on it either by default or because of old fortification potions, but remember, you cannot go beyond +2.

The old fortification will be recognized as a "stat" to the new system. So whether or not you got a stat or a skill from the old fortification, you cannot add a stat dye to it, otherwise, it'll replace it. So you should only add skill dyes to old fortifications, regardless of if it's a stat or skill unless you purposely want to replace it, and make sure it's a DIFFERENT skill if your old skill is already +2 since it will not surpass +2.

+2 to a specific skill is the maximum Lunia Gold you can have (applies to all items accumulation), meaning, if you have a +1 ultimate skill on a "back" item and another one on a "lower" item, that you should not add a ultimate skill dye to any other item, as you can never go past a +2. In which case, the max for Ultimate skill is level 3.