As you know already a revenge killer is the asset of the team in situations, Starmie is good with his speed and good versatility in typings.
Maybe swapping Starmie for my personal favourite (Yes I am bias) Gengar, he can use Thunderbolt will get rid of flying, water, and the main one, Hidden Power Ice Gengar can and should finish off a Dragonite if eved right. (Speed is the main priority in revenge killers because they are destined to die if they hit second) Shadow Ball with stab will do mass damage on Celebi, Focus Blast will kill off Scrafty.
As for Volcarona, they are a pest, but remember when there is a Volcarona you know its going to use Quiver Dance (or Butterfly Dance in US?)
Other then that, i cant really say anything bad about it because it is a good team all in all, just that you try use a revenge killer to counter your main weakness.
And i do agree Roserade is vastly underrated as a spiker, but i think it is down to its limited move list.