the way I've always seen it, the reason they wear the jump suits, is because x-com was set up in a hurry, they didn't have time to custom order some fancy uniforms so they just grabbed a bunch of surplus military jumpsuits and slapped a few patches on them. if they were to give them propper cammo uniforms they would have to order uniforms in a range of camo schemes (forrest, jungle, ice, desert, rocky, urban) that's atleast 6 uniforms per a soldier, and given how tight their funding is initially they probably figured it was cheeper just to give them the one grey jumpsuit which itself provides some degree of camoflage in an urban setting.
by the time you start making personal armour the need for uniforms has gone so x-com doesn't bother issuing uniforms they just measure people out and give them body armour instead.
as for the stats of the rookies, the way I understand it (from game lore) the only country really 100% behind the x-com project is japan (who took the first step in combating the aliens) the other countries tend to be providing balancing on the fence. if you do good they'll keep funding you, if you do bad they'll cut funding, if the aliens offer them a choice they'll switch sides, so the way I see it the soldiers you get are not the special forces you were promised but the dregs and trouble makers, (hence the poor moral, and inability to shoot the broad side of a barn at point blank range... and yes I've had a rookie miss a barn at point blank range) while keeping their best soldiers back to defend their own borders should they need to.
