Efficient use of (thumb)images

Hi everyone!,

I want to make a personal wordpress theme. On my theme I will need to following images:
catlist-post 50*50
The below 3 have the same ratio:
regular-post 310*175
grid-post 215*121 (is this smart? Should I just use the 310*175 and use css to show it up smaller?)
single-post 640*361

I know that wordpress make 3 images by default:
thumbnail
medium
large

I want to be as efficient so I like to use the 3 standard ones and adjust the settings to the appropriate size. But the only problem I see now is that you cant set crop for medium and large. Is there a way to activate that (I want to activate crop-mode for every image).

Would this be a problem knowing this theme is not ment for public and is only gona be used for my own closed website (where I got full control over the settings,…).