Well I think the separation of layouts to make them less coupled is the correct decision in general. However, I also believe, that the floating layout should be integral part of dwm and not be part of the way how other layouts are included. This allows that checks like lt->arrange == floating are valid.
Checks to other layouts in core parts of dwm will not be done and they are also not recommended in other layouts. Regards, -- Anselm R. Garbe >< http://www.suckless.org/ >< GPG key: 0D73F361