I don't think IB is fine for swing trading forex, the way IB works you are losing a lot in interests if holding overnight. They do offer currency futures though.
OP stated he was an equity trader.
The global account offered by IB allows one to trade multiple asset classes, most all global stock exchanges from one account. In addition, with a minimum $100K balance, IB offers Portfolio Margin, which allows flexibility and efficiency.
IB's financial strength, stringent risk management provides a strong sense of security.
All the above far outweighs the minimal interest fee's that may be accrued to me.
As you pointed out, Currency futures are an option as well, they of course have their associated costs. Roll over vs interest...
If a brokerage offers interest free margin, it stands to reason it will make up for it elsewhere.
Currency's are just one of the asset classes that I trade swing/intraday. Interest has never been a concern of mine.
That being said, one would be hard pressed to find a better, safer brokerage.