Personally I don't think it matters that much with such small amounts. I tend to use dextrose just because that's how I trained my brain when I first started brewing so if it's easy enough use that.
You can use malt, honey, dex, table sugar, fermenting beer and a whole range of things really.
I am interested in exploring traditional methods of krausening (adding fermenting beer) one day just because traditions interest me - my feeling is it will just make the beer fizzy like all the others.