Route 16 is short and leads to Route 17, which is a Bike Path. Once you start going downhill, you will keep going, even if you stop pushing Down on the Control Pad. You can stop to fish, but otherwise you have to move up or down the hill constantly. Route 18 is not downhill, so you can stop wherever you want to.
You'll have to use your bike on this steep road, and you'll whiz through it in a hearbeat if you don't sstop to smell the flowers (and catch some rare Pokémon). Other than the hunting opportunities and a few junior trainers, there is nothing of interest in these three small areas.
When Fire Pokémon Slugma evolves into Fire Pokémon Magcargo at level 28, it adds a second type (Rock) and its Defense nearly triples. It's still painfully slow (it is a snail, after all) and its Attack is too low to take full advantage of its Rock-type attacks, but it can do some serious damage with powerful Fire attacks like Flamethrower. Slugma only appears during the day in Crystal.
Pure Poison is actually a pretty nice type, since all you really need to worry about are Psychics. And Grimer and evolved form Muk certainly have the goods: Sludge at level 16 and Sludge Bomb at level 50 are pretty good attacks. But Grimer and Muk have only so-so stats, and with only Poison-type attacks, they have no way to deal with Steel-type Pokémon who are completely immune to Poison.