mutex and padding ?!

Using pthreads on a Linux platform, ordinary Pentium x86 guess 32 bit registers

(1)
a structure or array is shared but each single element of the array, or field of the structure, is not shared.

Example :
pthread 1 uses only Struct[0].field1 , Struct[0].field2 and Struct[1].field3
pthread 2 uses only Struct[1].field1 and Struct[0].field3

If any of field1, field2 or field3 has not size = multiple of 4 bytes the struct is not safe if I do not use mutex??

(2)
can I define mutex[10]
and let
mutex[0] be used by thread1 and thread2
mutex[1] be used by thread3 and thread4
etc. ?
If on a machine mutex has size not multiple of 4 byte will potentially threads3 and 2 for instance interfere?!

Thanks

Kind regards,