For Claydol, if you want to to be bulky, you gotta max out that low base HP before anything. Same goes with Tentacruel.
For Sceptile... I guess it's a nice idea to use Endeavor to deal with things like Blissey, but if you're running a SubPetaya set, it might seem like a better idea to just use another move for more coverage, I'd recommend Hidden Power Fire.
For Tentacruel, if you want to use him as a tank that does the job of laying entry hazards, why are you worried so much about its Speed? It isn't a sweeper. Tentacruel's base stat for HP is lower than his Special Defense base stat, so if you want to take as many Special hits as possible, max his HP, then add some EVs on Special Defense. Remove the 252 EVs from Speed and put them in HP. I also recommend you put in some EVs in Defense, because it's really weak, and use a Bold or a Calm nature.
Give Miltank Leftovers.