Cst sales with form c & form i

Dear Team,

I have doubt about CST sales with form C

The case is

1. We are purchasing Material from Karnataka under VAT and paying 5% on VAT on purchase

2. The same material sales to Tamilnadu under CST @ 2% with form C

3. At time of VAT Return we have 3% tax credit with us.

My doubt is whether can we adjust input credit with CST Sales @ 2%.

One more thing regarding Form I

We are selling material to SEZ unit located within states and outside the states.

We are collecting Form I against inter state sales to SEZ and my doubt the same whether applicable local sales (Whether we need to form I from local SEZ sales also) and if any document required at time of assessment as a proof of sales to SEZ other then customer purchase order.

Thanks in advance

 BAskar V