Yes, virtually all modern decoders are dual mode. They are meant to receive the signal overlay on a square wave AC signal, but they will accept and use DC automatically as they arrive from the factory. You can alter CV 29 to whichever setting your decoder manufacturer says will limit the decoder to AC operations (sensitivity) only.
"DCC-ready" does not translate to a standardized meaning from manufacturer to manufacturer. It is meant to suggest to the buyer that the engine is simple to convert to DCC operation by installing a decoder, but that is not true in the same way from engine to engine. Some require more work. Some have a pin connector that is not standard, for example.
To the asker, if you buy a "DCC-ready" engine, you will indeed by able to run it on your DC system without any modifications or caveats. This is so because it is not DCC, but is only DC capable when you get it, as the first responder has said. You must purchase and supply an appropriate decoder for that model, install it properly and safely, and then you can still use it on your DC layout, but you won't be able to modify motor performance characteristics via programming CVs. You can only do that with a DCC system.
