Is it possible to use only FSF software?

For the future, us free software advocates remain hopeful that it will become even easier for any novice user to install without hardware incompatibilities. That GNU/Linux has come so far in spite of so many obstacles is already awesome. Proper software drivers and firmware for video cards (except Intel) and the majority of wireless adapters remain the greatest obstacles to issue-free adoption of our operating system. I have not lost hope that someday, it will not be uncommon for computer stores to sell computers with GNU/Linux preinstalled.

Personally, I don't run any non-free software on my computer whatsoever. That level of commitment required me to purchase a compatible wireless adapter for ~$35. But a computer shouldn't have to do that to make his system run for whatever purpose he require (he also shouldn't have to pay hundreds of dollars for an operating system or an office suite, but that's another story). The hardware manufacturers, if they pretend to ethical, they will provide Free Software firmware and drivers for their devices, so that anyone is free to learn from them and improve upon them, passing on the improvements to all others in the community. If large numbers of us don't stand up for such freedoms, change will not occur. That's why we, knowing better, must push for free software across the board, rather than distributing non-free software as a matter of convenience or marketability, as Ubuntu, Fedora, Mint, Debian (to an extent), and many others have done.

I think, the answer to your question is yes, to both. It is possible to use only software that conforms to the FSF criteria. That's the idea behind Trisquel. Although sometimes you may need to get extra hardware, or change certain hardware, to ensure everything works.

Regarding computers with other operating systems installed. Using GNU/Linux has always been about trial and error. It relies on people like you, trying a particular GNU/Linux distribution on their computer, and reporting back the results. The latest computers are no different. I've found most varieties of GNU/Linux work well on new computers.