>RMS found it acceptable to use SunOS initially to create GNU.

Any source on that?

I know it from personal experience using GNU tools on Sun early on (really Solaris in my case, I wasn't quite that early a user), and I think from a talk or essay by RMS but for a moment I worried it might have been personal correspondence. Finding a citation seemed like a fun challenge:

https://www.gnu.org/gnu/thegnuproject.html

> [...] the easiest way to develop components of GNU was to do it on a Unix system, and replace the components of that system one by one. But they raised an ethical issue: whether it was right for us to have a copy of Unix at all.

> Unix was (and is) proprietary software, and the GNU Project's philosophy said that we should not use proprietary software. But, applying the same reasoning that leads to the conclusion that violence in self defense is justified, I concluded that it was legitimate to use a proprietary package when that was crucial for developing a free replacement that would help others stop using the proprietary package.

> But, even if this was a justifiable evil, it was still an evil. Today we no longer have any copies of Unix, because we have replaced them with free operating systems. If we could not replace a machine's operating system with a free one, we replaced the machine instead.

Still leave open the the question of RMS personally using SunOS (as opposed to some other proprietary unix) but I think at this point I'd just go dig up very old GNU sources for evidence of that, but I suspect your question was primarily about RMS' ethical reasoning which is well answered above.