I think you can get negative ranking only from beating the Serf Bot, which is very weak and only gets matched against beginning or low ranked players. I think all other bots have ranking high enough to guarantee positive ranking even to top ranked players (though not many points and you lose a lot whenever the bot wins).
Currently I'm lucky to be at the level where bots are decent opponents for me (I have around 60% win rate against them) so playing on Goko is great experience for me without having to cope with all the multiplayer problems. I hope they fix those problems before I learn to play better.
Any rating system that makes you lose points from a win, at least for someone with a non-provisional rating, is pretty messed up.
You could lose rankings on iso from a win.
I'm not sure about this actually. On the iso leaderboard, the uncertainty is increased per-day, not per-game. So on iso, the only way a win could decrease your level is if it decreased your mean skill, and that would surprise me (although I haven't checked the math to see whether it's possible).
To clarify, it's definitely possible on iso to only win games on a day, then see your level decrease. But that's because of the per-day increase in uncertainty.