Looking at the issue of a fleeing unit being formed or unformed, the front of the rule book defines a formed unit becoming unformed when the unit size falls below the minimum number of bases as set out in the rules (again in the first few pages). It does not mention anything about a fleeing unit being unformed. Plus, I think, though I could be wrong, that when a formed unit becomes an unformed unit it can not rally as per the rules, therefore by definition if a formed fleeing unit is unformed it can not rally, which goes against the principal of the rules.
Also trying to be a bit realistic, a mass formed unit running and being chased by a few skirmishers (with no other support) it is likely that the skirmishers will get their butts kicked, and would never risk running into a mass of enemy troops fleeing or not.
Interested to see what others think - Martin any comments would be appreciated, or we could discuss this at Swindon in a couple of weeks.