I'm aware that a US citizen may only have one margin account... is this the same with futures? If I switch brokers, and I can only have one, then that means no trading for a day or so :/
There is no rule against have more than one margin, cash or futures account. You can have as many as you can open and fund.
You can have a futures account with a US FCM and have a large number of sub-accounts, each with it's own account number and risk settings. This is how prop futurestrading groups work. Chicago FCMs do it all the time. I personally have mine set up like that at present.
Not so. That is just some ETrade bs (they are the only ones I know of that restrict multiple accounts). I currently have 2 futures only accounts, one fx only, 3 multi-asset brokers and 2 equities only brokers, all with margin. (each has their own function, some as backup brokers and some for swing trading). And to top it off, IB, one of my brokers, I have 2 different margin accounts - one sits on the US servers and the other on the Hong Kong servers. There is no restrictions on my trading activity. Your information is wrong.