I'm going by the definition used by Wikipedia, where Linux is described as an operating system, and the various variants are "distributions".
Don't let the term "distribution" fool you. Any distribution still fulfils the definition of operating system since it still is that piece of software managing hardware and software. Those distributions do use the same kernel and thus the same code but the settings (modules and such) can differ greatly so any distribution that deviates from the vanilla kernel considerably should really be seen as its own operating system. For example Oracle's Linux distributions. It uses Red Hat but they heavily modified the kernel settings so that it works for their own software and has some tighter security settings. The changes are so big that it makes it incompatible with the original they are based upon (and any of the others that are based on Red Hat). Of course with it being open source software you can always change the kernel for your own modified one.
What sets Linux apart from the rest is the fact that development is done differently. Where the rest of the operating system world is developing the system as a whole, Linux is cut up into pieces. With that I mean that i.e. Windows, macOS, FreeBSD, etc. are developed from kernel to applications by 1 team. In Linux the kernel is developed by 1 team, the apps by another and the distribution which bundles them all together is done by yet another team. That team does make it into 1 working operating system though and thus you should see it as such.
The BSD derived operating systems are much easier to understand since everything is done by 1 team. BTW, BSD is also a derivative of yet another well known operating system that now is a certification: UNIX. AT&T even tried to sue BSD and failed miserably (just like SCO did with Linux).
I was not aware that they weren't compatible with each other. It seems that I was mistaken!
While the BSD derived operating systems are quite easy, Linux is quite complex and also highly debatable (not everyone agrees with seeing a distro as a separate OS, they see any Linux distro as being Linux and will count them as 1 OS like you did). Can't blame you here.
The world doesn't end with BSD derivatives, Windows and Linux though. MS-DOS (and its derivates), Haiku (which might be better known as BeOS), NetWare, Solaris (and its derivates), ESXi, etc. are also supported operating systems that one can run in the VMware virtualisation products (they all share the same hypervisor which is why the VMs can be run on all their virtualisation software; this is also why VMware is a much more solid product than Parallels is, it not only has to work in their consumer products but also in their enterprise products which are far more demanding).
the problem exhibits itself better when resuming rather than booting a VM from cold. the issue is around how fusion restores state and kernel extensions that don't yield. but, to answer your question, the VM's are generally run two core with no more than 6gb ram. i find that adequate for most things as outlandish disk speeds make up for moderate ram settings. the MBP is 16gb ram, 1TB SSD, quad core i7 2.8ghz 2016 model. 2016 mac's have PSIe SSD's in them that support over 2gbps speeds. in short, they are **** hot. and when APFS hits the streets, they will be even quicker. so, i have plenty of "umph" with plenty of space.
