focus items

what is a focus item? Is it the thing that sits in the spot where a shield would sit? Can they be upgraded? If the item that sits in the shield spot is not a shield but an item that gives me +5 energy and +4 armor can i upgrade it without there being a requirement outside my pro/2pro?

I ask because i use a item in the spot where the shield normally sits only because i have my str and swordsmanship high for a great weapon i have. I don't use a shield because i don't have enough points in my tactics to use a good one.

Focuses are usually chosen for plus energy and mainly casters use them. Yes, it sits in the same spot as a shield would go. They can be 'upgraded' if it takes inscriptions (Nightfall items), the requirement to use the focus to its fullest remains the same.

Shields come in both tactics and strength requirements. You can look around for collector shields or craft one (expensive option). You can refer to:
http://gw.gamewikis.org/wiki/Main_Page
to see which collector shield you'd want or where to craft one.