I know that MB Trading allows you to trade minis.I am not sure for Interactive Brokers,but i think that they offer them too.Both of this brokers claim that they are not making the market for you.They have kind of ECN which means(in this case)that order routing is fully automatic without DD intervention.They are passing orders to other traders in their system.I do not believe that they are passing orders to true interbank market.
Other ECN style broker that i know for are Hotspot FX and Dukascopy.Whit them one trade just standard lots and minimal deposit is way higher than with MB Trading.Interactive brokers have also high deposit requirements.
Regards,
VTK