Well you've got a good start with the ork boyz, as no matter what style play you want with the orks you will need them. Gretchin are good meat shields and good at tying up enemy units while you move up your heavy hitters. I have 28 gretchin with 2 runtherders. What are you taking for your HQ? Weirdboyz are fun and I thought about heading up my army with one, but took two Big Meks instead as I have a walker heavy army. If you only have 500 points you're looking to work with I wouldn't take the gretchin. If you're looking for a horde army, get more boyz and a squad of lootas for heavy.
"There is a cancer eating at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has though and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us by instinct. We have given the horror a name to salve our fears; we call it the Tyranid race, but if it is aware of us at all, it must know us only as Prey."