Depends on whether you train or retrain. Retraining affects only Z3.par. Full Training affects all .par files. Of course you will normally still get the same backtest results, since you're using the same historical data.

I do not think that a different AssetsFix can have a noticeable effect on training. Except maybe in extreme cases, such as a very different Rollover, by factor 10 or so. In such a case the training would possibly generate parameters that close trades a bit earlier. If one of your AssetsFix values is completely wrong, you'll get bad results in the backtest. But the problem should be normally easy to spot in the asset list and the performance report.