I think sweetcorn are one of those crops that you can slot in whenever's convenient. Shouldn't think they need much manure and they're fairly drought resistant when they get going because they have long roots so maybe before or after the aliums?
I agree with BPP - it doesn't seem to matter where you put them. Remember though that they grow tall and close together so that they can cast quite a lot of shade so I always set mine out in a block on the northern edge of the veg patch.
John
The Gods do not subtract from the allotted span of men’s lives, the hours spent fishing Assyrian tablet What we observe is not nature itself, but nature exposed to our method of questioning Werner Heisenberg I am a man and the world is my urinal
I usually put the sweetcorn in after the broadbeans are picked in May - as far as I am aware corn does like manure so i add it and it likes the nitrogen from the legumes then i follow corn with winter/spring brassicas who seem to like the second hand manure or onions then i follow that with alliums or brassicas depending which way round the previous.