All great advice above. At 8 months we usually did not buy a separate seat for our children. But if you do then certainly bring a car seat. At that age the Cosco Scenera was the perfect travel car seat for us (it's comfortable, cheap, and super lightweight). Strap on some car seat wheels (e.g., Go-Go Babyz) and you're ready to roll!
Other than exit rows, there are no seating restrictions for infants. Some regional jets don't allow lap infants on certain sides of the plane because they don't provide an extra oxygen mask, but the A321neo (or any other plane Delta would fly BOS-SEA) will not have these restrictions.
And if you have SkyClub access, children under 2 are always free.