Result of ANDing & ORing functions expressed as CDNF & CCNF
There is no need to guess. Try to prove your guesses. If $f$ is the disjunction of minterms $\{m_i : i \in I\}$ and $g$ is the disjunction of minterms $\{m_j : j \in J\}$ then it is definitely the case that $f + g$ is the disjunction of $\{m_k : k \in I \cup J\}$, but not all $m_k$ are necessarily minterms. For example, $x + xy = x$.

Regarding $F+G$, try to use de Morgan's rules twice to reduce it to the case of $fg$, which is identical other than the operators involved. In Boolean algebra addition and multiplication are dual (via de Morgan's rules), so anything you can with CNFs you can also do with DNFs.