Well the vostroyans should be an exception as I have only 50 of them total. Seems a bit unfair that there may be a 6-men eldar squad and not this. Your call.
You dont have to paint a whole platoon each month, just a ten man squad, atleest thats my understanding. You could do a platoon over 3 months, then sprinkle in some other stuff.
I think he just means it has to be a force legal squad, eldar can have a squad of 6, imperial guard cant.