Right except: any of the rose comb breeds has a chance of carrying the not-rose gene, so they sometimes do produce chicks with single combs.
Yes, pea + rose is walnut, if the chick actually inherits those genes from the parents. But if the parents show pea or rose comb, they might not actually be pure for those genes. A cross of pea comb (carrying not-pea) and rose comb (carrying not-rose) can produce walnut, pea, rose, and single combs.
Rr is often written with the capital letter (dominant gene) first, no matter which parent it came from. But yes, a cross of RR with rr gives chicks that are Rr (looks rose but carries the not-rose gene that is needed for a single comb).
Yes, I should have clarified. Assuming thst the parents are homozygous for their comb types, what i said is true