Practically speaking, the answer is that wikisource's texts have terms similar (and possibly identical) to the public domain, although it is not always identical to public domain.
Sourcetext on wikisource is creative commons attribution share alike (BY-SA), https://creativecommons.org/licenses/by-sa/3.0/ ( https://creativecommons.org/licenses/by-sa/3.0/deed.es ) which means you can use any of it for commercial use. BY-SA is definitely one of the most lenient licenses -- aside from the public domain (where there is no obligation to give attribution).
For example, I publish a lot of my fiction under BY-SA, so people can make commercial use of my fiction (I don't care). The only condition is that it be labeled properly and that you identify the source of the text. If you take BY-SA content which is NOT public domain, then you are obligated to make clear that your product is also BY-SA. This can raise an issue if you are selling something which consists of BY-SA stuff and maybe copyrighted images.
I have solved this incompatible license issue by mentioning in the credits that the text is BY-SA but the images are copyrighted (or some stricter version of the CC license).
From an editorial standpoint, it makes sense to disclose that you have made some small changes (or big ones, if that is the case).
Glancing quickly over the material so far, it looks like a LOT of the works there are not just BY-SA but are actual public domain. Generally the designation is on the bottom of a wiki page. Although you shouldn't trust the copyright labeling all the time (occasionally someone mislabels something), but stuff from Wikipedia is usually accurately-labeled -- they police themselves well.