Answer to the first question: no, the sum is not always a monomial
Consider adding 2x and 5y. Both of which are monomials. Their sum is 2x+5y. We don't have like terms, so we can't combine them. The sum is a binomial as it has 2 terms.
======================================
Answer to the second question: yes, their product is alway a monomial
Going back to 2x and 5y. Multiply those two monomials and we get 10xy as a product. This is a monomial as it is one term. If p and q are monomials, then so is pq.