Lets say I want to short a stock X having lot size of 250 shares. But my position sizing only allows me to short a maximum of 200 shares.
Can I do so by shorting 1 lot in futures and buying 50 shares in cash at the same time?
Will I get a benefit in margin, OR will I have to pay the whole sum ie, Full Margin for future lot + Cash for 50 shares?
No, you will not get margin benefit. You will need full SPAN + Exposure margin for futures position and cash for buying 50 shares.
For your short futures position, you can get margin benefit by buying a call option. You can easily check margin requirements using margin calculator here or using the basket order feature.