There is no active ROM developer mailing list, so tba definitely is a better choice. There was some effort to clean up ROM with the RaM project.
Dead Souls sees active development as well (the main dev is a hero in my eyes for the amount of work he produces).
I would not recommend MUCK as the userbase is rather small. However that is not to say there isn't good work being done -- look up the user Valente on the code subforum of the wora.netlosers.com forum, as he's probably one of the foremost MUCK developers at the moment.
However if you thought that ROM was complicated I should caution you about tackling an established/canon codebase for any purpose other than getting a familiarity with mud servers. For actual development you may be better off with a barebones codebase such as NakedMUD (C/Python) or even something slimmer than that such as Socketmud (ports in many languages).
There are of course dozens of mud servers you can look at; all will be educational in some manner, but in the beginning stages it won't be obvious what is good practice and what is not. You may want to look up ColdC (similar to LP) and TeensyMUD (Ruby) to study. The author of Teensy, Jon Lambert, has a useful developer site up at http://sourcery.dyndns.org/.
However you'll find very experienced ROM and tba (i.e., Circle) developers at MudBytes, and I'll second Sam to say that is the most active mud developer site currently. It's a little surprising but in the last year there has been a significant growth in activity at MB. I think people are coming in from the fold so to speak and gathering at MB. There also is a good-sized code repository at MB as well.
Your other options are The Mudconnector which you already know, Top Mud Sites which has a somewhat smaller crowd of mostly developers (typically of established and long-running muds), and Mudlab, which is much quieter but usually with a good signal to noise ratio. MudGamers is an interesting new site with a fairly quiet forum, but a new approach to creating a more contemporary-looking portal for playing muds.
Not to be overlooked is the archive for the old mud-dev mailing list. There is a staggering amount of information to be gleaned there. The raw archive can be found at muddev.wishes.net/. Richard Tew also has done some noble work in combing through old usenet archives to find valuable mud development related threads, which you can find through his mud tag at posted-stuff.blogspot.com/search/label/mud.
I should note that many muds use the IMC chat network to link muds (MB has a portal to this as well on the front page of their site). Once your mud is running it can be useful to get on IMC if you're in need of real-time chat to fix a problem (of course, there are many IMC channels and you'll want to choose which one you use prudently).
Despite the fact that muds today are niche at best and unheard of at worst, there is no shortage of new muds in development. They offer a design and programming challenge that is still accessible to the solo developer, unlike any graphical game of equal size or complexity.
Furthermore you shouldn't be discouraged if it feels like you'll never release a playable game. Like many larger projects you may start and abandon it many times over, but you'll be building proficiencies across a wide spectrum of programming skillsets and applications -- not many projects will allow you to take such a whole systems approach. Good luck!