Choosing uintmax_t handles the integer case if I'm not overlooking something.
(1) Is there a similar such data type for floats, and, if yes, in which header? (it's not in float.h for me)
(2) is it correct that choosing a union of these 2 data types (assuming we can answer (1) in the affirmative) should always be most restrictive?
As pointed out in Eric's answer, C11 defines an object type with the greatest fundamental alignment in <stddef.h>: max_align_t.
Note, however, that this might not still work as expected on GCC. Taken from gcc's website:
A fourth version of the C standard, known as C11, was published in 2011 as ISO/IEC 9899:2011. GCC has substantially complete support for this standard, enabled with -std=c11 or -std=iso9899:2011. (While in development, drafts of this standard version were referred to as C1X.)
If you don't want to rely on C11 due to its freshness and current lack of support, I suggest defining a union with all integer types, all floating point types, a void pointer, and a function pointer: one of these must be the most restrictive type.