I have on nipple in between two panels and that nipple is occupied with feeder cables. My question is can i run branch circuits in with the feeder cable or do I need to install another nipple.
Yes. If the nipple is 24" or less, the fill is permitted to be 60%, see Chapter 9 Note 4 to Table 1. Derating also does not apply if the length is 24" or less, but be careful with this one. If you bundle the conductors together at each end of the nipple and you exceed the 24" you will have to derate.
Don