Let's not forget they were also developing tons of other mecha, aircraft, ships, new equipment, and weapon systems during this time period, they probably had every known weapons and aerospace manufacturer and developer working on something throughout 2000 to the 2009 period. So there probably wouldn't be a problem with them manufacturing and testing two different variable fighters.
As for the size of the UNS I'd say that would still depend on who was contributing troops to the cause, I'm sure most member nations kept their own armies and the UNS itself just started recruiting from those ranks and new enlistees.