12 or even more would be fine, I have 14 adults and 10 3-4cm in the same sized tank with red empresses, brown haps (15 of these), small dolphins, small albino auratus and a trio of peacocks..
Maybe your ice blue will be ok but I have found them to be ticking time bombs. I used to read articles on them thinking I had mellow ones until they turned lol Awesome looking fish though
Dems should keep to themselves they will give the odd chase to other tank mates if they come near their favorite rock etc but should be fine.