You can trade whenever you want to. No problems there if you have the materials, which you do. Just don't forget to trade back if you need to!
As for Earthquake, I would say one of the Nidos. Sandslash learns some pretty good moves by itself, and I believe that in FRLG it does learn that, too. Don't quote me on that...
As for another Ground-type you could use if you want, I would suggest Dugtrio of the three others you have listed. You don't have to trade for it (Golem), or train it to Lv. 42 just to evolve (Rhydon), plus Dig is a great move.
Hope this helps!
It's great to complete a Pokedex! So I'll need some help...
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