Steelix- true, is a very good pokemon for 2nd generation play, but, some of the things you stated are untrue.
1. Effective types: Fire, Fighting, and GROUND. water is neutral.
2. Steel type is not super effective against fire, it is actually not very effective.
Now aside from these errors, I do agree that Steelix is a good GSC pokemon. Unfortunately: PU will be based of of the DPP games. (referring to stats, move pool, etc.) So... Steelix is still a mediocre choice for your team, but if you are looking for a better defensive wall, I would suggest Forretress, Skarmory, or even Bronzong for the newer play. But, for your GSC team, I think Steelix is a nice choice. I do recommend a different move set though; Screech is unnecessary. Use Curse instead with his low speed, after one of these he is slow enough to Roar nearly everybody out, meaning that he can't be roared/whirlwind out. After he curses once his colossal defense is even better, and he has a high enough attack to do some massive damage with 100acc Earthquake, with STAB for a base 150Pwr. throw in a Rock Slide for his final attack and you got a solid Physical Wall with a great chance of taking out many teams. Good Luck with mine though