I think I'd start Foster over Stewart.
As far as trades, you're thinking correctly. You have a lot of depth at WR, so making a trade that strengthens either QB or RB would be great, depending on what's available in your league.
Help with mine?viewtopic.php?t=472412
12 team, H2H, Standard Scoring (Starters in bold)
QB: P. Manning, Luck
WR: Julio Jones, Welker, Alexander, Hilton, Shorts
RB: Rice, Wilson, BJGE, LeShoure
DEF: DET, SD