If Colon comes back this week but Adams does not, I'd say DeCastro starts at his natural position and Foster moves over to RT where he was surprisingly effective a few months ago.
If Adams comes back this week but Colon does not, then DeCastro could play out of his comfort zone at LG, but he should still be better than Legursky, who seems to be a disaster anywhere but at center and goalline fullback.
If neither Colon nor Adams comes back this week, I'd probably rather use Beachum inside rather than outside. Insert Beachum at Colon's spot (although #68 lining up at LG again gives me the heebie jeebies), then have DeCastro at RG and Foster at RT.
I was the biggest DeCastro supporter in the world leading up to the draft, so I'm excited to see what the kid can do, especially now that we really need him with all of the OL injuries. If he does indeed play at his usual RG spot, there is no better "throw him into the fire" test by having him face off against Haloti Ngata right off the bat.