Can Escallion be replaced by onion?
A little research seems to suggest that escallion is not scallion :

Escallion - The escallion (Allium ascalonicum L.) is a culinary herb. Grown in Jamaica, it is similar in appearance to the British spring onion, American green onion, Welsh onion and leek, though said by Jamaicans to be more flavourful. Like these others, it is a (relatively) mild onion that does not form a large bulb.
(from Jamaica Cooking Dictionary )

Now ... 'Allium ascalonicum' is not what most countries consider 'scallions' (that'd be the 'Allium fistulosum'); ascalonicum is what we normally call 'shallots' ... so it's possible that they have a variety of shallot (aka 'multipier onion') that's bred for the green tops.
A little further digging suggests that the Allium ascalonicum L. is specifically 'wild onion', which I know I have growing in my lawn (but only really harvestable in the spring, before I start mowing for the season).  It's possible that there are regional variations, but I'd say that wild onion tops are closer to chives, but that might also come from my harvesting them before they've fully matured.
...
All that being said, I substitute things all the time.  I'd probably cut down the amount of onion (or use a milder red or yellow sweet onion), go with chives or shallots, or as @belisarius mentioned, go with leeks.
update: and in walking through my yard today, I noticed that the wild onions are back up above the grass, so at this time of year, you might have a free (and closer) substitute -- if your yard has areas where it looks like densely packed chives, and if you cut some, it'll have an onion-y smell, then you most likely have wild onions.