I'm a 3 railer: Should I order a 2 rail car?

2R trucks will run on GarGraves track just fine.  However, when they go thru Ross switches, the 2R wheels will occasionally have difficulty traversing switches due to the wheel flanges not riding thru the frogs properly.  If they are in the middle of a consist moving forward, they are frequently fine - but they will usually derail in backing thru a switch.

It depends.    As mentioned above. 3-rail standards for flangeways and frogs are grosser than 2-rail, so derailments may occur.   On the other hand with your curves, if you have decent turnouts, you should have no problems.

As to couplers, Atlas O cars and Weaver cars come with Kadee compatible couplers.   Older Atlas couplers will couple fine, but are hard to uncouple.   The new ones are Kadde knockoffs.   

Most all 2 rail cars made recently that come without couplers come with pads drilled and tapped to mount Kadees.   And many come with mounting screws.   

If it is a car you like give it a try.   Worst case, you have to replace the wheel sets.

If your track is "flat top" (Gargraves, Atlas, MTH, FasTrack) then the car shouldn't have a problem. There are caveats:

• Curves can't be too sharp. 36" radius (O-72) is usually the minimum with some exceptions. PS-2 hoppers and other cars less than 40 feet can usually handle 27" radius (O-54) but cars longer than 60 feet usually require 42" radius (O-84) or better.
• Turnouts, as mentioned above, can be problematic in some situations because of the larger flangeways. Curve-replacement turnouts (O-72) can present problems when backing scale-wheeled cars through the diverging route. #5 turnouts seem to do alright but #6 and above create another problem because of the extended length of the frog. We put a "frog point" into a #8 curved turnout to support scale-wheeled equipment.
• Placement within the train can be more critical. Light cars with scale flanges on tight curves WILL create a string-lining problem. Car weight, wheelbase, and weight are more critical on sharper curves (36" radius/O-72)-- less so on very broad curves (72" radius/O-144).
• Once you start buying 2-rail cars, you may not be able to stop, even if you don't go 2-rail. Even worse, you may start converting your 3-rail cars to scale wheels as well as Kadees.

Hope this helps some.

Order some 2-rail trucks and put them on a 3-rail car.  If you don't like the way it runs you're not out of a lot of $$$.

Or buy a 2-rail car and a set of 3-rail trucks, if you don't like the way it runs replace the 2-rail trucks with the 3-rail trucks and you have another piece of rolling stock in your "collection".

Many thanks for the knowledgeble replies. Unfortunately, my immediate plan to purchase a 2 rail Atlas O car foundered on an old bugaboo . . . unreasonable shipping charges to Canada ($35 for a $66 car and no combined rate from the seller for a pair of cars either).

I think I will try Bob's suggestion and get a pair of scale wheel trucks with my next order with a seller who has them.