I'm pretty new to software and word lists as well, so I have some of the same issues. I have both CrossFire(XFire) on a mac and Crossword Compiler(CC) on a PC. XFire uses the .dict file format for its list, CC uses .lst format. I have done fills with both programs. I much prefer CC for word list management.
http://beekeeperlabs.com/crossfire/docs/index.html#MAINTENANCE is the page where the XFire default dictionary is discussed under "Dictionary options". It is ranked, but imperfectly, on a scale of 1-100. A few other details are discussed here. One thing that annoys me about this list is that it includes some bogus plurals, i.e., an 's' is added to words that are never used as plurals. It also contains a plethora of drug names, plant genuses, etc. that could never be used as fill.
So to answer your first question davidm, the XFire default is not very good. I am assuming the XWord list is the one from XWordInfo available to Angel members. It has to be better than the default in XFire. Just convert it to a .dict file and add it to XFire using the Dictionary tab under Preferences. You can enable or disable any of your word lists there. You can also change the order and designate a primary dictionary (word list). You can't edit the default list in XFire, but you can locate it, make a copy, rename it and then edit that file. Personally, after viewing it in a text editor, I don't think it's worth the hassle.
The default dictionary in CC is also also a ranked list, but I'm not sure what the ranking is based on. I get less bogus entries using it than I do with XFire's default. In CC you can also add the XWord list and convert it to a .lst file. You can also merge it with other lists there. You have many more options in the Word List Manager in CC than you do in XFire, e.g., you can choose whether to preserve the rankings of lists that you add in or use the rankings in the list you are adding to. The Pro Filler you get with the pro version has more options than the grid filler in XFire, but I think the 'fill from theme' option could be improved.
Be careful when merging word lists. Make up backups and rename them to avoid messing up your originals. Also, most lists use different scales and ranking standards, so you already have some choices to make before merging. It is important to decide which file is being merged into which (order is important!).
Finally, the Jeff Chen personal word list at XWordInfo is supposed to be the gold standard and has stuff ranked in detail based on his vast experience. He explains his ranking methodology under the FAQs here
https://www.xwordinfo.com/WordList/FAQ.aspx
