Unit graphic tiles are automatically scaled to the tileset tile size by the game itself.
So it's possible to used 18x18 Ironhand's unit graphics with a 16x16 tileset, and everything will show correctly as 16x16 tiles.
But tilesets are not so flexible.
Every tile in a tileset needs to have the same dimension as they are all included in the same 16x16 grid.
Also, the raws contains references to tiles, and those references are not the same in all tilesets. ( Creatures are not at the same locations, mineral walls are not categorized the same and are also located differently, etc. )
So to add different tilesets (Ironhand, Mayday, etc.) to the assembler would be a lot of work, it would require supporting a configuration system to support the different graphic sets options for Mayday/Ironhand, it would require supporting different tile sizes, it would require splitting the Mayday/Ironhand tilesets into parts in the same way I did for my tileset, and it would require a way to synchronize specific tileset options with their raws.
One last thing, this time a request. Seeings as though now you can support multiple gfx sets, is there any chance that the beefmo set could be supported? I love his set, but too intimidated by raws etc to attempt adding them myself. (this is thrins beefmo repack and recent updates to it http://www.bay12forums.com/smf/index.php?topic=54399.msg2218513#msg2218513)
I will try to include beefmo's units in the next update.
You can add unit graphic options to the savegame manager through the /data/config/graphics/ directory.
You will need to create a new subdirectory then add a version.txt and raws.zip (preview.png is optional). Use the pre-included graphic packs as a reference.
(Yes, the util now uses zip files to store it's data.)