I did a cross reference on my insurance web site (Humana). I put 500mg of Metformin with 1000mcg of Chromium and it said there was no interactions found. You might call your pharmacy to make sure though.
I also did a search on
www.drugs.com this is what it said, pretty technical.
metformin and chromium picolinate (moderate Drug-Drug)
Description:
MONITOR:
Pharmacologic dosages of chromium (>= 200 mcg/day for adults) may potentiate the hypoglycemic effect of insulin and oral antidiabetic agents. Chromium appears to play a role in normal insulin function and glucose utilization, and some investigators have suggested that it increases insulin sensitivity and glucose tolerance and decreases blood glucose levels in certain diabetics, possibly those with low chromium levels. However, others have not corroborated these findings, and a few have even reported a negative effect on glucose tolerance and blood levels.
MANAGEMENT:
Until further data are available, therapy with pharmacologic dosages of chromium should be administered cautiously in patients receiving insulin and/or oral antidiabetic agents due to the potential for additive risk of hypoglycemia. Patients should be monitored for changes in diabetic medication requirements.
Metformin and Chromim 
