From your description the Corid has already done what it needs to do, so it should be discontinued now.
The thiamine in the feed should not be an issue, most feed has vitamins added in and the Corid dosage should account for it. It's adding extra thiamine on top of that that could be problematic.
Personally I've never found the need to do anything different after treating with Corid (other than stopping of course). It's well tolerated by most birds and shouldn't affect their health adversely unless you were overdosing them with it.
The diarrhea has to be something else, whether worms, or too much extra stuff (like water rich treats can thin out poop).
The thiamine in the feed should not be an issue, most feed has vitamins added in and the Corid dosage should account for it. It's adding extra thiamine on top of that that could be problematic.
Personally I've never found the need to do anything different after treating with Corid (other than stopping of course). It's well tolerated by most birds and shouldn't affect their health adversely unless you were overdosing them with it.
The diarrhea has to be something else, whether worms, or too much extra stuff (like water rich treats can thin out poop).