Give it a try, tuna in oil has been a successful inclusion of many carp anglers spod mix and normally what works for carp work for bream and barbel (especially if its fish based), plus there ain't much chub will refuse.
As for the floating oil slick, I wouldn't worry because some will become dispersed through all the layers of water.
My advice regards adding to your groundbait, is liquidise the whole contents of tinned tuna along with twice as much hemp, then thoroughly mix with your dry crumb, making sure there are no lumps, leave for 5 minutes and mix as per usual groundbait...