I don't think this is nearly universal - certainly none of the systems I use have /tmp as tmpfs.

/dev/shm, on the other hand, is almost always guaranteed to be a tmpfs on glibc systems.

No.... some, sure. Not most.

RHEL7, and all it's derivatives (CentOS, Oracle Linux etc.) don't use tmpfs for /tmp by default, and they are arguably the most used linux distros in the world.

But with only 1 GB capacity

It depends on your setup. The default when no size is given is to use half the RAM.

Half? That seems kind of excessive. Are you sure that’s right?

I would guess that it’s given that capacity, but the memory is allocated lazily.

Correct; tmpfs only uses as much memory as as is used by the files it contains.