I wouldn't really use HP grass in that set. It doesn't beat Suicune or Starmie anyway, and you'll get worse sleep talks if using it. And the PP issue. Rhydon still takes some decent damage from fire blast, and ttar will get worn down with spikes down eventually. Not that HP grass does "a lot" more to ttar anyway. Just make sure you have a solid answer to ttar (and rhydon) such as suicune, miltank or vap.
Talking about moltres, another interesting set would be Fire Blast | Sunny Day | Reflect | HP Grass/Flamethrower/Rest/filler @ leftovers or charcoal. It can be scary if they don't have suicune/starmie/blissey.
As for good offensive combinations, I'll start with a pretty straightforward one, Machamp + Cloyster. Cloyster can take out Starmie or Zapdos with Explosion, two of the best Machamp counters (especially starmie), while Spikes will help wearing down other Machamp checks, such as Exeggutor and ghost-types. You are not keeping Spikes vs Starmie though (unless maybe you have Toxic and a lot of patience), but you can at least take it out.
The limitations? Machamp's well-known mediocre staying power, and the possibility of mispredicting Cloyster Explosion or not geting/not being able to get the desired target with it. Also, the fact that if you are running into triple+ counters such as Skarm+Starmie+Zapdos or just Egg+Zapdos+Spikes machamp will have a hard time.