Will I have to be face penalty if I sold stocks from my holdings and bought other stocks on the same day for delivery worth 80% of the previous sold amount?
You can use the 80% sale value received to purchase other stocks, there will be no penalty.
Is this valid for mutual funds too?
When you redeem a Mutual Fund, the sale proceeds are credited by the AMC to your trading account only after settlement.
To purchase mutual funds, you can use the sale proceeds from the next day onwards.
So next day I guess you mean T+1 day?
So on T+1 day - we can use only 80% or all 100% we can use to buy MF?
Yes, on T+1 day. You can use the entire sale proceeds.