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For definitive information regarding Parisian public transportation please refer to: http://www.ratp.fr/index_eng.htm (the French site is more detailed).
The Carte Orange, generally speaking, is the least expensive and most customizable pass, available for all zones and zone to zone, monthly (Mensual) and weekly (Hebdomaire). Pricing: http://www.citefutee.com/informer/tarif_abonnements_o.php and additionally a passport sized photograph is required. It is important to realize that the Hebdomaire is valid Monday through Sunday, sold only until Wednesday of the current week, purchased after, not usable until the following Monday. That being the case, the Orange may not be most appropriate for your needs, and you should consider the Carte Paris Visite http://www.ratp.fr/ParisVisite/Eng/index.htm Both passes and individual tickets are available at any Metro or RER station (including the RER station at CDG).
Most economical is usually a combination of pass and individual ticketing. Most attractions are within Zones 1-2 (Paris to the immediate suburbs), excluded are the airports, Versailles, Disney most obviously which are Zones 4 and 5. However, since one is likely to travel to and from the airport but once or twice, and Versailles is likely a onetime roundtrip daytrip it is unlikely that the extra expense for a pass including those zones will be justified.
As far as being refused a Carte Orange, one might imagine a frustrated ticket agent’s difficulty in explaining the niceties of the pass to a non-French speaker, preferring to sell the simpler, but more expensive Carte Paris Visite; but with a little persistence, some French, and the required photograph one should have no problem.