I agree that Phoenix tabs are more bigger, with maybe more details particularly for the solid tabs. But you can easily play with "phoenix_tabs_small" or "phoenix_tabs_medium" to adjust the height you want. Also, the dirty bottom bar in Phoenix is much larger that Nil Theme... It's a choice, like the default tabs height, i'm on large screen and my goal is not necessarily to win 5 pixels on the height of the tabs. Although I maybe slightly reduce the height of dirty bottom bar by a few pixels for greater rendering on small tabs.
In all cases, the Nil theme is very well designed and it's beautiful too
Note that i have added a mention and a link to your theme in Phoenix readme.
On Clouds Midnight theme, I change almost nothing ^^, compares the two files with an app like kaleidoscope to see the differences, but they are not numerous, maybe two or three colors.
You can absolutely use the original theme or any other in fact, "Tomorrow Night Eighties" rendering good too. On "Tomorrow Night" shipped in Phoenix, I only changed the background color.