I'm not using Arch at the moment, so I'm not sure, but I believe you do not have to update all packages at the same time (unless some are required for other packages updates) and usually such changes are tested by the community before being released widely... but if someone else knows better, please correct me.
You can stop certain packages from being updated [0] but the wiki explicity says it's "unsupported" [1] so probably best to update everything and hope nothing breaks.
New packages for the "core" repo, for example the Linux kernel, are first introduced to the "testing" repo, which isn't enabled by default. You would have to manually change your pacman.conf to use it.
From the wiki [2]:
"After a kernel in core broke many user systems, the "core signoff policy" was introduced. Since then, all package updates for core need to go through a testing repository first, and only after multiple signoffs from other developers are they allowed to move. Over time, it was noticed that various core packages had low usage, and user signoffs or even lack of bug reports became informally accepted as criteria to accept such packages."
It's usually no problem to keep a package or two from upgrading via the --ignore argument to pacman. Arch calling it "unsupported" doesn't mean it's inherently risky, it mainly comes from the fact that you can shoot yourself in the foot with it if you do it to something like glibc. Keeping a userland program like Chrome or Gimp downgraded will not screw up your system.
