Depends on the type of effect you want, Xylophone, Glockenspiel and crotales these 3 have a clear attack sound, it means every single note of the tremolo will be noticed, it won't merge smoothly as perhaps you expect, for instance Vibraphone would much better for this, you can use the same vibraphone soft mallets on xylophone and would sound smooth just not loud, in glockenspiel you perhaps could find some of the hardest mallets of timpani those with small head and use it in glockenspiel that would do a nice effect but again, will be mich reduced in volume, for crotales you would special mallets I don't know how to call them but it would require a very small head like 7mm and made of a sort of not too hard plastic like the rubber in bicycle breaks, so the Xylo+glocken+crotales can work fine with some modifications, but insist vibraphone would be much better and perhaps celesta, you also bow the crotales but will sound like 5 seconds only.