Looking closely at some photos and videos, it looks like the recoil spring on the XS doesn't go all the way to the end of the slide. It looks like they're using the same spring for all the models.

Is that the case? Are they using the same length spring on all the models?

All Dagger (non Micro) use the standard Gen3 Glock RSA (or the PSA equivalent).  

The X slide is essentially a G19L slide.  Putting it on the S frame gives you a Gen3 G47. Putting it on the Compact frame gives you a Gen3 G49.

One story I have read is the reason they haven't had any X slides for sale is because they have it been able to get barrels.  But if they sold the slide by themselves, we could use our own Gen5 G17/G34/G17L barrels.  

The first thing I have always done with a complete slight assembly is to sell the barrel and RSA.