As a mainly Tyranid player, I suggest only a few heftier models. Spend your points on lots of the cheaper units. Opponents either spread out in an attempt to take out everything you throw at them, or they'll concentrate on taking out your big guys and leave themselves open to flanking by your mass of ground troops. as I said though, keep in mind that I typically play 'Nids.
As a side note, you don't happen to know where I might find the (discontinued) Boxed Army of the (discontinued) Steel Legion do you? I've been kicking around the idea of using them to make a Genestealer Cult.