Well the Pokemon that you suggested are half and half mainly because they can easily be shut down or KO'd by many other great leads.
Ferrothorn and Scrafty get shut down by TopThundy. This is probably the most feared doubles combination in the whole doubles metagame as they can shut down or cripple almost any lead with taunt and fake out
Also, Terrakion and Weavile shut you down as well as Close Combat can KO Ferrothorn, whiles brick break will do some damage to Scrafty. WHen playing doubles, many Pokemon that are very good in OU usually become very useless in doubles. Suicune, Umbreon, Vaporeon are part of those Pokemon as they are all rendered useless when hit with taunt, which is a very common move in the doubles metagame. I am not sure if you mean which of those Pokemon go well together or if you want Pokemon that can support each other well in doubles...anyhow, I will give you some good Pokemon that form a great pair in double battles
TopThundy
As I mentioned above, TopThundy is probably the best pair, but also the hardest to utilize, so you will only see VGC or extremely skilled doubles battler use this lead. However it is not invincible and can be taken out by various other good leads. Anyhow, TopThundy is made out of Hitmontop and Thunderus-I. This lead will often cripple any other lead with relative ease with the following moves
- Taunt
- Thunder Wave
- Fake out
beware for they don't only cripple but hit back very hard with their powerful STAB moves
Gastrona
Gastrodon and Volcarona also form a remarkable lead. This mainly because Gastrodon can take any water move that is aimed at Volcarona whiles Volcarona can whipe out grass type Pokemon for Gastrodon meaning that this lead will often stay around for a while if used wisely. Bug Buzz will scar Tyranitar whiles scald will be denting and crippling many sweepers, fun non?
Scraftnoir
As Scarfty is one of the few Pokemon you listed is that is actually a star in Double battles, I will give you a nice partner for him. This partner should be Dusknoir. With access to Trick Room, Dusknoir will make Scarfty fast enough to start doing some serious damage, including denting Volcarona, 2HKOing Amoonguss, and 2 to OHKOing Tyranitar who forms a threat for Dusknoir. Under Trick Room, Scrafty is faster then Tyranitar which means that Tyranitar will no longer be forming a threat. owever be wary of TopThundy who will usually fake out Scrafty and Taunt Dusknoir, making this pair useless.
**Cresselia can also be used over Dusknoir, however it is weak to the mighty Bug bite that Scizor runs, something that Dusknoir can handle better
Excados
**
Excadrill and Zapdos, a very interesting lead. As we all know, a Choice Banded Excadrill is one of the most feared Pokemon in the entire game, mainly because of the powerful Earthquake. Zapdos will be immune to Earthquake as long as it doesn't use roost. It can cripple wih Toxic, hit Scizor with Heat Wave and kill of Politoed with Thunderbolt, why wouldn't you use this lead. Discharge can be used over Thunderbolt since Excadrill is immune to Discharge anyways
Metoir
Metagross!!! Metagross is one of the best Pokemon to use in double battle due to the presence of it's high attack, speed and defense. With Explosion, it will often be OHKOing almost every Pokemon if not every Pokemon on the field. Those that survive will be baring a severe wound and can be finished of by the rest of the team. Dusknoir is also a top doubles Pokemon. With it's immunity to explosion, there would be almost no better partner for Metagross. It can set up Trick Room whiles cripple with Will-O-Wisp and finish of low health foes with Shadow Sneak. Devious should be the word that you are looking for
There are many other great leads, however I will not mention them all as there are infinite. These are popular, but who knows what hidden power lies in the land of the doubles?
