Swarm unit of Imps has base of 40mm, so it does not fit into 25mm door. Shall it be able to pass the door as it is actually swarm of smaller units?
Technically no, but i house rule this type of thing prior to the game starting. Usually i agree what size models can fit thriugh any doors etc and abvuously they have to be clear of the door once they have finished moving.
If they engage someone that would mean they stop in the door then i would just slightly rearrange the models to fit.
The official rule though would be that it doesnt fit (and so no need to slightly move anything to accomodate)