I was loosely following this game as it was developing, and I'm pleased that the town played the game based on suspicion rather than outguessing the setup. It's tempting to make game decisions based upon that, especially in situations where it seems unlikely a mod would choose such a role combination.
I can understand Juls' perspective on this, but in the interest of developing and enforcing guidelines in an objective manner (or as close to it as possible), Mafia Neighbours should be a viable role option. The key defining element of a Neighbour is their unconfirmed alignment, so a neighbour flipping scum
shouldn't
confirm the rest of the neighbourhood as town. Meta has slowly evolved to see more roles given to mafia; Mafia Trackers, Watchers, Gunsmiths etc. are all becoming relatively commonplace (for better or for worse) - so, unless it was explicitly outlawed, a Mafia Neighbour pair shouldn't have been seen as impossible, even if the role was somewhat redundant.
This is the sort of reaction I expect when the first Mafia 1-shot Vig gets used. Although technically a viable role as well, it strongly betrays town's trust in this role's meta, as their whole belief system is calibrated from only seeing town vigilantes before. I'm not sure you can reasonably forbid this role from being given to mafia, much in the same way you can't really forbid two mafia neighbours. The only reason you could forbid something like this, is if you believe it's acceptable to ban a mafia-aligned role if it has never been given to scum before. There had to be a time when Mafia Tracker and Mafia Watcher were first used, and I'm sure the town would have felt equally hard done by in those situations, because it's something they couldn't reasonably expect based on meta.
Disallowing something because it's never been done before only serves to stifle the evolution of the game. I think if two Mafia Neighbours ever get used again, it will go a long way to causing doubt over the validity of neighbour pairings, much in the same way a Tracker claim doesn't necessarily confirm you as town any more.
I hope this explains my position a little more - I'm happy to answer questions about this, though.