The Summoner has 2 pieces; Tr and S1. His overall graphic animations(tr) are 1-piece, not requiring any other animation pieces to show his arms, head, etc...It's the S1 folder that makes things weird w/ swapping his animations.In my experience it is the second set of mode columns that seems to make swapping monsters buggy. I personally don't like the way animations are structured in there. What I do when token switching like this is view all the animations with Cv5 and adjust and swap them out similarly to how they are originally structured. This oftimes requires animation editing, copying/pasting additional frames, etc...Sadly, there's no tutorial to explain this(I don't think). What I started doing was just playing around with it.
How does this help you? I'm not sure if it does unless you feel comfortable editing aniamtions. There are a lot of great people in the Keep(esp. in Gfx and Anim Forum) that are really helpful. My advice is to stick closer to matching mode token switches and/or start editing away! 
Hope that helped at least a bit 
(edit: spelling error)
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To ask well, to answer rightly,
Are the marks of a wise man:
Men must speak of men's deeds,
What happens may not be hidden.
-Havamal(Words of the High One)
[This message has been edited by Har'lea'quinn (edited 30 April 2002).]