I'm trying to create two mappings which are efficient for myself:
map X ddp
Which I'd use to delete and paste in one go.
map X "_dw
Which would delete a word without yanking into a register.
However I don't want to break any existing, useful shortcuts so I'm wondering what keys I could use - any suggestions? Am I being too uptidy?
Uhmm, no, don't. When creating your mappings try not to overwrite anything ... not so much because you don't use the command you're overmapping, but because some plugin which you have/or will have maybe using it.
And then you overmap it, and then you have to worry.
Personally, for commands such as you gave as an example, I like Ctrl+some key combinations. There are a lot of free ones in vim, and the letters on the left side near Ctrl make a nice pair then.
Btw, what are you trying to do with those mappings ... I understand the second one (delete word by word), but the first doesn't make sense to me. What is it supposed to do ? Transpose lines ?