So SR+Spikes+Leech Seed is now legal on Ferrothorn but is it even worth it to run it? You're basically forced to run either Gyro Ball or Power Whip in the last slot so you're not set up bait. And it becomes difficult set up all those layers and start seeding things.
No it isn't. I can think of 0 reasons to run 3 support moves, 2 of them being entry hazards. Very few pokes can set-up dual hazards, such as Deoxys-D, and this is because he has insane bulk and Recover, or is used as a suicide lead.
Ferrothorn cannot find time to set-up both hazards, and if you make him to, his role of walling will be seriously compromised, with only 1 attack and no T-Wave/Protect. If he had Recover, maybe he could get away with it, but as of now i say that i would never want to run SR + Spikes Ferro + Leech Seed Ferro.
The problem with Ferro is, that while it is a fantastic wall and hazards setter, it lacks reliable recovery and can easily be set-up on. So you will have to cover those 2 aspects when choosing Ferro's moveset. This is why Leech Seed is almost a staple on every Ferro i choose, because it gives you a way to recover damage, while also making you harder to set-up on, as almost everything hates a Leech Seed on the switch. Then you pick your hazard of choice, and you have 2 slots left. Those 2 slots should be focused in making you more difficult to set-up on. This means that one attacking move + T-Wave, or 2 attacking moves, or even 1 attacking move + Protect are the best combinations of moves for Ferro, imo.
I also find Bulldoze + Gyro Ball to be a very potent 2 move combo, paired with SR/Spikes and Leech Seed. Let's see what Power Whip hits harder than Gyro Ball in OU, shall we? It hits Cloyster, Donphan, Hippowdon, Dugtrio, Jolteon, Gastrodon, Gyarados, Jellicent, Keldeo, Politoed, Reuniclus, Rotom-W, Starmie, Tentacruel and Vaporeon.
Now let's take out the pokes that would never stay in against Ferro, or come into him: Gastrodon, Jolteon, Vaporeon, Strmie, Rotom-W and Politoed. Let's also remove the pokes that don't care about Power Whip for various reasons: Jellicent (WoW + Taunt beats you anyway), Reuniclus (Power Whip isn't enough to prevent CM variants from setting up, same for TR, and also why would you let your Ferro into Reuniclus is beyond me) Hippo (wouldn't stay in against Ferro as Ferro sets up on him and drains his life with Leech Seed), Dugtrio (dies from Gyro Ball too), Keldeo (Keldeo would prefer to avoid setting up on Ferro, as T-Wave, Leech Seed and Power Whip all cripple him or kill him, and you can't come into Keldeo, as at +1 it ohkoes with Secret Sword).
So that leaves us with: Cloyster, Tentacruel, Donphan and Gyarados. Out of those 4, Cloyster would never attempt to set-up on Ferro, so Power Whip only matters when you come to stop a +2 Cloyster, at which point, most likely you will have already been weakened enough for Cloyster to get past you, or else why would Cloyster set-up in the first place? Also Donphan would never come into Ferro, as it fears Leech Seed and Power Whip, and Ferro wins one vs one against him, with or without Power Whip.
So that leaves us with Tentacruel and Gyarados. Even Tentacruel is questionable, as it is very likely to burn you, and is not afraid to stay in and Scald you a couple of times, as unlike most waters, it is neutral to Power Whip and gains 12.5% life each turn. So, i would prefer to avoid staying in against Tenta most of the times. So that leaves us with only 1 poke, where Power Whip is really useful, and that is Gyarados. So unless your team fears him, or you are afraid of Gyro Ball running out of pps, Bulldoze is better than Power Whip as a second attacking move, as it prevents Zone from trapping and setting up on you, prevents Volcarona from gaining any speed boosts against you, which means that if you have a poke with more than 100 base Speed with strong physical attacks, you will be able to handle her after Ferro dies, and finally and most importantly, makes you a SubCM Jirachi counter if you run enough Atk evs to always break it's Sub (192).