Tilly was cast too old, they were going to have to do this sooner or later. It's kind of like watching Snape, Lupin, and Black in Harry Potter played by actors much older than their characters' mid 30s.
I assume you meant Detmer and Owo. Nilsson hardly had any chance to showcase Sara Mitich's talent, same with Rhys and Bryce. I want to see more of Nilsson, Rhys, and Bryce. I think we could have a good friendship too in Rhys and Bryce along the lines of O'Brien/Bashir or Harry Kim/Tom Paris.
And how many times in media do we see a black and Asian guy hang out? It's not common in western or even Asian media (only ones that come to mind are "Rush Hour" and the lesser known tv series by Sammo Hung "Martial Law"). We need to see more of Rhys and Bryce.