No it cannot.
breed it with a ditto or a venusaur of the opposite gender
venusaur!?
Of course. All it needs to do is have sex (Yummy!) with a female venusaur.
Try using a Ditto, if you have one
Venusaur is the final evolution of the line beginning with Bulbasaur. Venusaur cannot evolve into another form. It can, however, breed (at the Pokemon Daycare) and produce an egg which will give you access to an earlier evolution. Venusaur can Mega Evolve into Mega Venusaur using the Venusaurite. Mega evolution, unlike regular evolution, is temporary, only happening during battle.
A female Venusaur and any grass or poison type male.
To get a Bulbasaur egg you need to breed a female Bulbasaur, Ivysaur or Venusaur with either a Ditto or another male Blubasaur, Ivysaur or Venusaur. This is the only way to get one legitimately.
No you cannot breed with any of the legendary Pokemon even if you use ditto
Errr... Slowpoke comes from slowpoke.
Venusaur does not evolve.
You can't catch Venusaur on Firered, but you can get Venusaur by choosing Bulbasaur as a starter then training it. You can trade it (from someone who chose it as their starter) or you can use AR or Gameshark to catch one. If you know someone who chose one as their starter but don't wanna give it away you can ask them to breed it with a ditto and give (trade) you the egg. It will hatch into a bulbasaur =)
Slowpoke evolves into Slowking by attaching a King's Rock to Slowpoke and then trading the Slowpoke to another player.