Hello

do the internal Allies have to comply to the more archers than spearmen/javelinmen rule?

It seems almost impossible to do with the combined  TUGs and have a playable army

The "The number of javelinmen and spearmen cannot exceed the number of archers." requirement is for the army and not by (internal) contingent I believe - as it doesn't mention contingents 

If it did apply to internal contingents I'm not sure I'd agree that it'd make it almost impossible to get a playable list; not to mention the internal allies are optional so if it were it'd be self inflicted 

if one internal ally is taken, does it mean that 2 horse units have to be taken?

Yes. One for the main army and one for the internal ally as there is a minimum of 4 bases for the list. Each of those units could be just 4 bases of course.