I know quite a lot of boys with the surname Butler:
William, Hal, Aiden, Lucas and Thomas.
But, tbh, I think Butler is a surname you can put most things with without any problems at all. I'd possibly avoid A names so you don't get 'A.Butler' but then I do know two A.Butlers, one male and one female, and I've never thought of it as an issue until right this moment. And other names that can be a job like Tanner. For a girl I wouldn't have Iona but then you wouldn't use Iona with any surname that's a noun.
I think Cassidy goes with:
Ashton, Ayrton, Callum, Conor, Cameron, Dylan, Dexter, Dominic, Damien, Elliot, Ellis, Evan, Hayden, Hunter, Harrison, Jesse, Jasper, Jude, Jake, Jamie, Kieran, Logan, Milo, Morgan, Nico, Nathan, Owen, Quinn, Rowan, Robin, Reed, Rio, Tate, Zach