the top UK tyranid player (at least in EpicUK tournaments) only uses a pair of harridans in his tyranids, the rest is all swarms of small stuff, termagaunts, hormagaunts, genestealers and gargoyles, plus with some creative conversion work you can make damn good proxies for the bigger beasties from 40k kits, like
these or
theseThere are decent tau proxies in the DRM range, the kraytonians have some small skimmers which work as pirhanas and tetras, their flyers would work as barracudas too imo... depends how close you want the models to look to the GW ones really....