I'd be hesitant to use any brass fittings in any coolant connections. There was an End Wrench article that warned against using coolant flushing machines. There were several reasons given. One was the possibility cross contamination with copper ions if the machine had been used on a vehicle with brass or copper radiators. It could cause a reaction with the Subaru coolant.
in November 2008 Subaru approved the Wynn PowerFlush III coolant exchange machine since it uses three tanks that allow it to perform a clean water flush but it seems to me the warning about or copper ions would still apply.
I'd go with aluminum, stainless steel or plastic rated for cooling system use.