Yoo! Congrats on your job offer. I work in consulting myself, but based in SEA. Most of my colleagues in the US offices have the SPG Amex card.

Although flights are paid for by their respective offices, a small portion of them choose to pay out of pocket first via their CCs (and accrue points for miles) and get reimbursed later. I do this too.

As for choosing a ff program, as others suggested, I'd stick with one carrier for each program.