Mactjaap, TopsTerm was very unstable on 6kKB ROMs on an original Mac 512K. The Telnet functionality was OK for a few minutes, particularly if you stick to a single session, but some Unix apps have very poor VT100 emulation (see my screenshot for Lynx elsewhere). The FTP client was very flaky.

I have corresponded with a few of the original authors of NCSA Telnet and TopsTerm about obtaining source code. All were very helpful but were unable to find a copy.