hm hm hm hm.. im not too good at giving advice but here goes,
the tau as far as i know are shooty heavyish but not so great at CC, so i dont think you will have tooo much difficulty there. other then just getting close, i advice using venoms and such.
as for tyranids, depending on how big the battle is. you might be fighting some monsterous creatures and lots of numbers of small lil tyranids, i suggest ummm possible using Harlequin kiss to guarentte making wounds in, since the Harlequin have a high WS ability it shouldnt be too hard to hit them.