I think the biggest challange would be to cut the Ytong block exactly to right side to make a snug fit. It is very difficult to cut it exactly to size as when cutting you usually shave of a bit more. Small ants can fit into some small gaps. If this happen to you just fill the gaps with cotton.
If you had in mind an ant display farm with 1.5 cm width then I wouldn't use ytong. I think you'd brake a few bloks of ytong before you'd suceede. If you are planing to put it in basin than it is not a problem.
Ytong is not that hard so I don't think that just by puting it in you would scratch the glass. However, procede with caution.
