Usually i indeed broker have own rules regarding with spread, but usually if trader prefer as scalping trading hence will seeking broker that offer low spread, this giving benefit
What matters, for scalping, is to use a broker who genuinely executes trades on your behalf in the interbank market, and not a counterparty market-maker pretending to be a broker, who holds the other sides of your trades (and your deposited funds, and makes up their own prices and the rules governing all the transactions!) and is trading against you. Obviously enough, a genuine broker doesn't mind clients scalping the market, but a market-maker doesn't want clients scalping them.
Why market maker not allow scalping whether this will making them being bankrupt or there are certai another reason ?
If you take IB for example, spot is slightly cheaper on transaction costs but worse on o/n rolls. For shortterm trading i prefer spot, but if you hold a few days or more futures are definitely the way to go, o/n swaps in retail are really expensive. Also in futures not even all G10 pairs are tradeable due to illiquidity.
Some boker now offer free swap account, and this will giving benefit for trader that like with swing trading which usually will hold order more than a day, usually this type account for islamic account but many trader non muslim also choose this option