What are the best "file bit depths"settings you use?
 
Mine are:
Record Bit Depth : 24
Render Bit Depth : 32
Import Bit Depth : Original
 
I found some songs I did a while ago where the instruments were all recorded at 24 bit and then when I recorded vocals (or maybe when/if I rendered or saved them) they are 32 bit
Could this cause issues?

The settings are reasonable. Clips at different bit depths are no problem.

64 bitfp all the way. Think about it...whilst 64bitfp won't fix all errors that are caused by things like fx processing...it goes a long way to solving this problem. 64 bitfp is a must if you don't want to lose any virtual analouge emulation type effects. Again it's about processing effects and errors that are introduced when processing these types of effects...you also lose a lot of bottom end definition when you use anything but 64bit fp throughout the digital signal chain.

Its actually not a good idea to set 64bit as the default bit depth for record and import since you are wasting space in most cases. Unless your audio driver supports 32 bit or 64 bit streaming there is no advantage to higher than 24 bits.
Import should always be set to "Same as original"
If you are always working in 64 bit mode then a 64 bit render bit depth may make sense.