Layers pellets are their main food and actually they don't need anything else except greenstuff such as cabbage, plus any wild greens they find or short grass. Long grass isn't good as they sometimes take in long strands which block their crops. (The crop is like a stomach bag about halfway down between the head and chest area, and when a hen has had a good meal it fills up and you can see and feel a bulge.) mixed corn is OK in very small quantities, as you say it's a treat, useful to get them to come to you to be hand fed and tame, and gives them extra warmth before bedtime in winter, but it's fattening if overdone - not more than a small sprinkle per hen per day. Avoid human food treats, except some fruit eg they will go mad for grapes, Apple or pear cores, or off cuts of your own cabbage greens or salad veg etc. Cooked human food contains harmful fat, salt and sugar, they love it but it's bad for them.
Aubiose is excellent on the coop floor and in the nestboxes. Are they in an enclosed run, and if so, how big is it? They need a minimum of 2 sq. metres of run floor space per bird if in there for very long each day, so a freestanding coop is better than one with a small run attached.
Several threads on here about building coops or adapting sheds etc, do take time to plan carefully and ask advice about design from some of the experienced coop builders on here. You might also consider the advantages of buying a plastic coop ready made, they're excellent in many ways, hold their value well, and much less prone to the dreaded red mite than wooden ones. Have a look at Green Frog coops, fof 6 hens you would need a Medium Chicken Lodge. The sizes and design might help you to plan your own coop, even if you didn't buy one., I got mine from here, customer service was excellent and delivery very fast http://rarebreedpoultry.webs.com/greenfrogcoops.htm but there is a Green Frog main website as well. They say a max. of 5 larger hens or 9 bantams but I've had 6 normal hybrid-sized hens in mine quite comfortably.
I cannot believe how the previous owners could have just left you with 6 hens, no food, and no information! At least they've now struck lucky with you two.