I am afraid there is only one club you can support – and that is Manchester United.
You are breaking football supporters rule #1 that once you are hooked on a team, no matter for what reason (glory hunting, uncle supporter, whatever) you remain a supporter of that team for life. End of.
You simply cannot change horses mid-stream no matter what.
And whatever team you wish to join now, may I suggest, will not want you as a supporter if you formerly supported another team. (especially United!).
This is how football works in England.
And all that about football hooligans was way back in the 90s, it simply doesn’t happen any more. Just about anyway. I struggle to remember the last time there was trouble at Old Trafford. And I am sure the very occasional minor scuffle that might happen, happens all over England.
That video you posted was some silly joke.
When I was last in Manchester on a United match day I saw a City shirted fan come into a pub with mostly United fans in it and no notice was taken at all.
You need to go to Russia nowadays if you want to see the proper hooligans you talk of.
However, I feel I need to correct something in your OP. Manchester United might have the city's name in their name, and their stadium is indeed in Manchester, but they're not really a Manchester club. People from there support City. The typical Man Utd fan hails from Surrey or Kent - well off, conservative counties south of London.
This is mind bogglingly wrong. Maybe in percentage terms there are more United fans from the south but that is only because there are so very, very many of them outside Manchester.
But in numbers, United fans far outweigh City fans in Manchester itself.
This from the Mirror is not absolute proof of what I say, but is pretty indicative:
If you add up the City and United shirts that were sold in Manchester last year,[2014/15] an enormous 95% were red - just 5% were sky blue.
(And this is despite the fact that only 10% of United shirts are sold in Manchester!)
http://www.mirror.co.uk/sport/row-zed/your-teams-supporters-really-from-5018590