The Sa'Vasku ships would be great as Tyranids, with all those spikes and sticky out bits. The Phalon ships might work for Tau? I've never seen pictures of the latter's ships, though their surface vehicles seem to have a lot of sweeping curves, a bit like the larger Phalon ships.
Yes, the Sa'Vasku ships look the part exactly. I have opted for phalon weaponry though, as that whole energy allocation stuff reminds me too much of SFB.