Regarding the first question: Then you use something else instead of that component. For instance, the existence of networkd does not preclude the use of (say) dhcpcd or your own network setup scripts. Of course, there are a few components that are more central (e.g. journald, udevd). In that case you file a bug report and/or fix it. Same as when one component of the kernel (which is vastly larger than systemd) doesn't work for you.

As for the second question: certain parts of systemd can certainly be used on non-systemd systemd (such as udev or nss-myhostname). But most would require at least some changes.