A short answer: Intuiface’s ordered collections, such as Asset Grids, use a unique size for all children items to provide a 100% control on the size & position of items.
With a mixed layout, you can’t predict what the collection will look like before feeding it with the data.
An alternative solution would be to use a free-layout kind of collection, such as a Scroll collection, and position your images in X/Y/Width/Height. We have users generating such layouts using Excel files as a data source, and entering these 4 values for each elements there.
Else I recommend you to start a new thread about your collection suggestion in our #wishlist category.