I'm new here but maybe I can add something to the list? It looks interesting.
Dustox-Definitely C, maybe B depending on how you use it.
+Poison/Bug typing is very good in the earlygame which throws many grass and poison types at you.
+base 90 Sp Def at lvl 10 means this thing is tanking any special attack in the earlygame. 60/70 for physical bulk isn't bad either.
+Its level up moves are very good. Venoshock at level 20 means it can leave a dent in Julia's team with support from base toxic spike Trubbish. It learns Light Screen, Moonlight, Bug Buzz, Psybeam, Quiver Dance. All of this by level 40!
+Psychic/Bug/Poison coverage is very good for the early parts of the game. Good against Fern,Cain,Victoria,Florinia,Corey,Taka(!),Aya,Luna.
+Quiver Dance by level 40 means that it can setup against Serra and Noel when the base 50 Sp Atk starts to fall off. Light Screen support is very good against Shade.
+It is a surefire counter to PULSE!Tangrowths, especially since its typing and ability Shield Dust prevents it from losing SP Def from Acid Spray.
-Wurmple's evolution is random, so it might take a few tries before getting Dustox.
-BST of 385 is terrible for the mid to late game.
-The rock/psychic weakness really hurts. Bad against Kiki(all of her mons have moves that check poison/bug types),Radomus(some psychic moves obtain a part rock type in the field) and Shelly(the only bugs it can hit for neutral damage without an SE counter are Volbeat and Illumise).
-Base 65 speed really hinders it in the midgame.
Slaking- B tier
+BST of 670 as a Slaking is very scary. Every stat has base value of 100+ except for Sp atk at 95(still good) and sp def at 65.
+Truant isn't much of a problem in singles when your opponents are suffering OHKOs thanks to the base attack stat of 160.
+One of the few pokemon in-game that learn counter by level up.(Not sure if it can survive Solaris' Garchomp to OHKO it).
+Access to moves like Covet(good for stealing Heart Scales and other nice items), Hammer Arm(Great coverage), Play Rough as a Slakoth at level 38!
+Vigoroth has base speed of 90 so its outspeeding most mons in the midgame. With 80 in HP/Str/Def , it can pack a punch.
+If you are trying to get play rough, the easiest way to get there is to drop Slakoth at the day care center. It learns all of its good moves from level 20-38. You should get it by Kiki/Aya.
+STAB Strength is still great for neutral damage since its coming off of its monstrous attack.
+Good coverage means its versatile against most gym leaders. It can even land an OHKO against Samson's pokemon with Play Rough or Counter.
-If its spending time in the daycare, that's a hit to its availability.
-Truant is terrible. But it hurts more in double battles, and situations where Slaking can't OHKO the opponent.
-While it does have access to TMs, most TMs in the early to mid game aren't very good.
-Really low Sp def is bad even though it has very good HP.
-Struggles against Kiki, and will generally struggle against anything that resists Fighting/Normal. Like Shade.
-Although its time as a Slakoth is short, its pretty bad.
