A Single grinder is going to draw a lot of fire pretty quick (they always do) as everyone is afraid of Phlegm. I might try and split my letters up into a few squads of 8 (khornes magic number) and use the leftovers as a squad of 6 to guard the grinder depending on where he goes. I only played one game with one grinder ever, and it got owned the turn after it arrived. I run two or none now, and usually princes.
I guess I'd do prefered: Thirster, a squad of 8 letters (should you break them up), one squad of horrors, squad of fiends and the prince.
Secondary: Everything else
I know you're going to see some people say they like bigger groups of letters etc, and yeah they work fine, it's all just situational. I've had pretty good luck with 8 squads. You just really have to play them smart and get the charge so you can inflict maximum pwnage. Though I typically play squads of daemonettes wich have assault/defensive grenade gift, and then a squad of letters to clean up after the charge. Daemonettes are basically invuln save fearless genestealers, and pretty much win combats. Of course there is a couple strategy articles on this very subject here, good read if you haven't looked at them.
I would stay away from dropping the grinder until you have other stuff for your enemy to worry about, because it WILL become primary target.
But yeah, play the army a few games vs different opposition and see how you like to run it. Standard stuff applies, don't charge your thirster into a squad of lightning claw/hammer termies and expect to live, etc.
This has all been just my opinions on Daemons, which I play (with fairly decent success) and love. Let us know how you fare