You'll only get Budew as an egg if the Female Roselia/Roserade is holding a Rose Incense. Otherwise, you'll get Roselia which can be evolved to Roserade with a Shiny Stone. But if you do that, you'll lose out on Extrasensory, which is a good addition.
Anyways, everyone else stated my thoughts on the team but I'll add a bit of other stuff.
For Slaking, since it is in the Field Egg Group, you can try to use a Smeargle Breeding technique (check Etesian's Breeding Guide / Recommendable Sets) to get TM Moves on it such as Return, Earthquake, Giga Impact, etc.
Etesian put moves that you could breed for Darmanitan, Archeops, Durant, and Scrafty in the link above.
Gyarados is pretty much alright, try replacing Aqua Tail for Waterfall whenever you get and you can try to run Bounce as an option if you feel Ice Fang isn't working out.
I'm trying to get this physical moveset for Venusaur at the moment, but I'm missing a mon so I won't say it yet. Anyways, Special Venusaur would just be Giga Drain and Sludge with option of two other moves you want. To get Sludge Bomb onto Venusaur: Grimer>Stunfisk>Wooper>Snubbull>Roselia>Bulbasaur.
Also, be noted with Episodes that Gen VI Breeding Mechanics (any episode after 13) will allow you to be breed Egg Moves from the parents if the baby can learn it, starting from the Mother to the Father.