It is common practice in engineering to use analogs; things which are not the same as the real thing but similar enough to be able to gather some useful data.
Yes, but you have a lot of work for selecting the right analog and document where the differences will influence the results.
A block of lead can be a useful analog for making a mass dummy of a nuclear bomb, but it is not useful for measuring the radiation.
And for Ares I-X, there is actually only visual similarity to the current concept of the Ares I. The aerodynamics aren't even precisely the same, though at least similar enough for serious comparison.
But the flight dynamics, that should get tested, for example by turning off the RCS every 10th second, are not the same. You have no sloshing, no fifth SRB segment, and different internal structure than the real upper stage - all factors which influence your roll behavior a lot. Even the separation sequence data can't be used. A upper stage with 95% liquid content behaves completely unlike a stage with 100% solid rigid structure,especially when you examine staging. Especially when you have massive vibration loads, which will induce motion of the fuel.
The recovery of the first stage will also be different later, since the masses and staging sequence will vary.
If they would take the business serious, Ares I-X would be what Ares I-Y is planned to be: The first test flight of the Ares-I rocket, in the Ares-I configuration.