Help regarding Rhydon

1) Mud Slap for sure. It has a lot higher DPS and will outperform Rock Smash even when Rock Smash has super effective bonus and Mud Slap doesn't. There's really no scenario in which Rock Smash would be better.

2) Stone Edge for defense for better coverage and for offense I think both are equally good depending on situation. You might want to have one of both for offense if you're really pushing it but If you'd just have to choose one overall I'd say that Stone Edge is better. Earthquake does slightly more damage but it has longer cooldown as well making it harder to dodge incoming attacks when using Earthquake.

3) Not really. You already have a level 30 Rhydon with great moveset. Even if it has low IV's, the level 23 one would be only marginally better (that one doesn't have very high IV's either). Your Rhydon squad looks fine as it is for now, that should be enough to choose from for almost every situation. I myself don't use Rhydon much because I usually have a better Pokémon to use anyway, so because of that I wouldn't recommend powering up more of them for you unless you get one with high IV's and good moveset.

4) Not really.

Tips against Gyarados:
Well, Gyarados has double weakness against electric, so Jolteon is going to destroy any Gyarados pretty easily. Gyarados does have a pretty weak quick move for defense so I don't think beating Gyarados should be a big problem. Problem with Rock is that even though it does super effective damage against Flying type, Water attacks (Gyarados' Hydro Pump) will also be super effective against Rock. For that reason I'd say that either use an Electric type or just one of your otherwise strongest attackers (Dragonite, Vaporeon or even Snorlax).

For Gyarados I'd use an electric instead - Jolteon is probably your best bet, but Electabuzz or even a highish level Raichu will do.

I prefer Stone Edge to earthquake - it hits hard and fast, but Earthquake is fine.
Mud Slap carries STAB so is a better move than Rock Smash. Rock Smash as a fighting move is okay against Snorlax though.

I wouldn't invest candy in any of your options - hold off until you get a mud slap/ SE (or EQ) and IVs 75%+. In the meantime your high level Rock Smash ones should be fine for fighting Snorlax (or Chansey).ones

1) Both of the fast moves are slow, I don't like using Rhydon very much.
But between the 2, MS is the best to me, it has STAB and can be usefull in different matchups.

2) I prefer the animation of SE, but EQ is a nice attack too.
I think I would choose SE, only for the pleasure to see the animation^^

3) No, investing stardust in Rhydon makes absolutely no sense to me.
Rhydons only has CP, it doesn't make him a pokemon you are over using.
Even for the matchup he is supposed to be the best by now (against snorlax) a Arcanine kills it faster.
The only reason why people invest stardust is because they are fooled by CP, but trust me, Rhydon doesn't deserve it.

4) As you have 5 rhydons and some others that you didn't mention in your post, I guess you find pretty easily rhyhorn around your area.
Don't invest stardust in this pokemon, just wait untill you catch another lvl 29/30, evolve it and use it if it has the good moveset (MS/SE), no matter the IV.
Then you can compare both your Rhydons.

And PLEASE, don't put this bad boy in a gym...

I was prestiging a gym which had very nice defenders and because people are using 2 accounts to kick the last pokemon and put theirs inside, the gym has now 3 gyarados and 4 rhydons.
PLEASE don't do the same thing, Rhydons are really a freaking joke like gyarados as defenders...

As said above : to destroy Gyarados, use Jolteon. A 2100 Jolteon beats a 2700 Gyarados without dodging. If you dodge, it s even easier.

The only reason for Rock Smash / Earthquake is Tyranitar
The only reason for Rock Smash / Stone Edge is lapras

OK people are getting out of control with the Rhydon bashing. I guess it's the cool thing to do now.

Whatever, Rhydon is a beast. Mud Slap/Stone Edge is an amazing move combo. Yes, mud slap is slow. But who cares? Just avoid water and grass and you'll never have to dodge another quick move again. Rhybro shrugs off attacks like little goldfish nibbling on his hide.

Stone Edge is the 2nd highest DPS charge move in the game AND it gets STAB bonus. Not to mention the fact that it has the lowest cooldown of any full bar move in the game. Plus it has amazing type coverage. You can use it vs. anyone. It's only weak to 3 types: Fighting, Ground, and Steel. Awww, shucks. Guess I can't beat up on all those Magneton and Primeape defenders. Boohoo

Yes, they're easy to beat with water or grass mons. But as one of up to 10 defenders in the gym he absolutely can be part of a well-rounded team. I just put mine in to defend in a gym where there must be 7 Gyarados. Figured it would help round out the coverage.

Are the people putting 7 Rhydons in a gym dumb? Yeah. But so are the people putting 7 Gyarados. It doesn't mean he can't be a good defender as part of a well-rounded group.

Also, did I mention he's a complete, total, 100% beast on offense?

(Also, to answer your question, don't ever drop dust on a Pokemon that's got 60 some % IV's unless it's Lapras or Snorlax and it's the only one you have. You'll get a better one someday and then you should level it up to your heart's content. Mine's level 35 and I have no regrets.)

none of them -- rhydon sucks.