Rainbow trout are less piscivorous(fish eating) than brook and particularly brown trout. When they get larger, they do add more fish to their diet. Rainbow trout, particularly those under 12" tend to feed heavily on invertebrates, like midge larvae, other insect larvae, scuds and insect hatches. When I clean my rainbow trout I rarely find fish in their stomach but the browns many times have fish in theirs.
Most ponds I have worked on that have trout even in heavy numbers will still have healthy populations of fathead minnows. Some sources even recommend not stocking any forage minnows as they directly compete with trout for food sources.
Rainbow trout of all the trout species are the first to take pellets. It may just take time for your trout to realize that pellets are food. If you bought your trout from a hatchery, they in all likelihood were at some point pellet fed. If they have been in the pond for a while and not pellet fed, they may take a while to relearn to pellet feed...
If you can, take some pictures of these minnow species, I am curious what species you have in your pond...
