When you set up a custom game, there is a window where you can supposedly configure the other players (Number/AI/Human/Custom Race ... I haven't managed to edit the entries of this window.
Is this a bug, or am I missing something?

Are you talking about the 16-bit Windows games which needs 32bit Windows XP or older to function properly?

Where are you playing it, on some retro-PC?

I presume it is not sold on GOG, at least I didn't find it with a fast search...

https://en.wikipedia.org/wiki/Stars]

Now that's a name I have not heard in a long, long time... used to play the shareware version back in the Win95 days, but I can't remember enough to help, sorry.

First question: yes. I'm playing Stars! (full game version 2.7i) on a modern Win10 PC (you can use DOSBox For Windows or another 16-bit emulator or VM to run it), and. And it's not sold on GOG.
I was curious to see how I like Stars! today (still very much!), and standard games work well. Custom race generation and launching custom games work, too, except when I want to change the players, where the game won't let me change anything.
I'm hoping to find players on this GOG forum, who not only remember Stars!, but also have some experience with setting up custom games - why not give it a try? I'll ask the same question on the Stars! Autohost forum, too.

I really thought this was going to be a thread about Resident Evil 3....

... and I thought Stars! (with an exclamation mark) was speaking for itself. :-)

Or possibly PCem - I've been having fun with an old copy of Windows 98 and an emulated Pentium 233 MMX (which is totally a good use of my ryzen 3700X)

My experience with dosbox is that if it breaks, it'll break in a way that usually crashes the program. The biggest problem with dosbox is that it doesn't support long filenames. I doubt this is a problem for Stars!, though. I'm not familiar enough with the game, nor do I have it, however my guess is that the issue is PEBKAC.

EDIT: To be clear, i simply think you forgot the controls.

Stars! runs just fine in Wine. At least, the 26i version is running fine.

This thread is not about running Stars!, but about changing players and races in custom games.

Is there a legal way to get this game free?

Yeah, I was able to do that.

No, but there are not-illegal ways to get it, considering it is Abandonware.

Are there any contributions to the thread topic?

Yes, if I remember my US laws correctly, the only way of legal way of acquiring abandonware is getting its original release and ripping it to your PC or other platforms for strictly personal archival purposes. Acquiring abandonware through other means like websites, even if you own the original release, is illegal in the US. The exceptions are if the work was released to the public domain by the original owners or its copyright has expired (death of author + 70 years).

Of course, Americans can decide to acquire abandonware through these means and likely suffer zero consequences given their nature. But you're SOL if a copyright holder does decide to come after you.