FCP does not handle mixed resolution or frames rates without rendering any mismatch. The question is always what is the deliverable.
There is no magic setting that will handle this. If you need to stay in HD you will need to convert all the SD graphics to HD and 24p, not a good idea. Or if your graphics are the most important you need to downconvert the HD and add pulldown to edit in a 30p (should really be 29.97) to match the graphics.