Just in case you/they were suggesting to use the same controller for both functions, don't mix vSAN and VMFS used for vmdks etc. on the same controller:
The VMFS vmdk wouldn't be stored on vSAN and thus wouldn't be an Object and thus vSAN wouldn't really care about it - sure it would show as <shrug> under VM > Monitor > Policies > Hard Disk, but other than that I can't see anything about it being particularly broken.
That being said (and of which you seem to be abundantly aware) having multiple storage types as dependencies for a single VM to function isn't a great idea - I am not an expert in backup solutions but it seems like most of our customers use methods that collect the backup data locally on vSAN and then move it somewhere else (though not necessarily via all just one VM with multiple storage types attached).
To answer your other question - yes you can apply whatever you want varying Storage Policy to each individual disk on the same VM, and technically it is feasible to do things even beyond this like making a snapshot chain with each snapshot having different Storage Policy than its parent