Yes, this is probably true and the reason is most probably license issues. A "rescue CD" will have to hold a copy of a working kernel and this is perhaps licensed software which one cannot simply distribute or copy. The Linux kernel is free and you are entitled to copy it as often as you like, but this is probably not true for SCO Unixware.

A "generic UNIX Rescue CD" is impossible. "UNIX" is not a single, coherent system but rather a standard how an operating system has to do things or how it is supposed to react to certain circumstances. We can talk about "cars" and every one of them will have a driving wheel, the accelerator pedal right, the brake pedal left, etc.. So in a sense all "cars" are equal. Still, this will not mean that one can build a "generic car brake", which you could use on any car, regardless of the car being a Maserati or a Beetle.

If you compare UNIX with, say, Windows, the difference is: if a certain program compiles on a Windows system and you transfer the binary to another Windows system it is guaranteed to run. (in principle! You may not have enough memory to run it, not enough disk space to store it, etc., but apart from such issues it will.) In UNIX the situation is very different: UNIX runs on many different platforms, different architectures - systems which are vastly different. A binary compiled on one such platform will almost never run on another platform. But you are guaranteed to be able to transfer the source code to another platform, compile it there and it will run.

Lastly: boot Unixware from Linux. For the reasons sketched out above it will most probably not be possible. It may be the same hardware architecture they are running on but internally - executable format, library calls, .... - they are probably very different. You might be able to mount the Unixware filesystems on a running Linux, though, as this article suggests.

I hope this helps.

bakunin