Hi timmy, from your homepage I see, you play mainly with Bachmann and LGB stuff. These do not couple very well with some couplers of other makers. They got their knuckle couplers mounted very low to the trucks. You started, fixing a bolster under your cabooses end. Why don´t you simply fix a piece of stripwood to the complete lenghi, to make it a wider bufferbeam. Underneath, at the same hight as your other knuckle couplers, attach one of these with the bendable head. They move almost 180 degrees sideways. If this is not enough to get around the small curves, fix the screw of the bufferbeam lightly, so the shaft can move as well.
I hope my model mock up on the 600 mm radius track is clear enough. I did not feel to get two cars ready, just to proof simple things like couplings for tight curves are possible. Have Fun Juergen Zirner