You are able to use Pokemon with more than one types. So Aron is a go. Why didn't you ask about Geodude, he being part ground and all, just curious.
Because Aron is Steel:Rock and Geodude is Rock:Ground, thought the order might have had something to do with it, anyway thanks for clearing that up, no time wasted training him up. :)
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Trainer Name: Greeny
Badges: 2
Play Time: 3:58
Latest Progress:
Gave Steven his letter or whatever it was.
Trained Aron and Geodude to level 15/18 while searching for Nosepass (turns out you get him from using rock smash so I'll go back for one later)
Beat Brawley, just.
Sailed to Slateport, beat all trainers and got some soda pop.
Beat the Magma members in the Museum and gave that Captain his "Goods".
Got a Rare Candy from the Trick Masters house.
Team:
Rocko / Geodude - Level 22
Moveset:
-Tackle
-Selfdestruct
-Magnitude
-Rock Throw
Steely / Aron - Level 20
Moveset:
-Cut
-Metal Claw
-Mud-slap
-Headbut
EDIT
Made more progress..
Trainer Name: Greeny
Badges: 3
Play Time: 4:50
Latest Progress:
Made my way up route 110 and beat May.
Arrived in Mauville City and got the Mach Bike.
Trained on Cycling Path and Route 117.
Evolved Geodude to Graveler
Went back to Mauville beat Wally and Whatson.
Team:
Rocko / Gravelar - Level 25
Moveset:
-Tackle
-Selfdestruct
-Magnitude
-Rock Throw
Steely / Aron - Level 24
Moveset:
-Cut
-Metal Claw
-Mud-slap
-Headbut