I am looking to get a set of 4 steel wheels for my base G6. I have the (PF9) 16" cast aluminum, painted wheels on it. Correct me if I am wrong but these are 16" x 7" wheels that have a 5 x 110mm bolt pattern.

Will a 16" x 7" wheel with a 5 x 4.5" bolt pattern fit?

What other size wheels will fit?

5 x 4.5" wheels won't fit a 5 x 110mm pattern. The 110 metric corresponds to 4.33" SAE, which I'm pretty sure doesn't exist.

Now, if your 5 x 4.5" wheels have oval holes (multi-fit), they MAY fit.