The basic rules/points/stats for most of the species can be had for free, it's the more detailed rules that are in the books. So you can try out the different sides before committing to the books. B&B is, as you say, the central Europe powers. Dwarfs, dogmen, rabbitmen &c. Catalucia is the Orcs/Goblins/Elves, Deadloque is the Undead/Elves. The 4th book "Grapeshotte" is about artillery & specialist units.
Each of the first 3 books has the rules system in it and so can be used standalone.
(I don't know much about Slaughterloo. Sorry.)
We have found that FL forces of around 300 points make a reasonable game. We can play 2-on-2 with those in a club evening if people are experienced or learner games of 300 a side.
300 points gets you around 6-12 figures (I did manage to get 15 in there for my Dwarf militia who are low quality & hence low price.) I do tend to include an officer, a standard or a musician in those groups for the variety and so it's at least structured like a proper unit. You can also include a mix of skill levels and character traits. Most of the FL range comes in "unit packs" which make a good basis for these units.
The starter set has 6 orcs and 10 elves in it, and will give you two about-300pt forces; and they're nice figures. The "shilling starter" packs include character sheets, miniatures and scenarios.
If you're interested in seeing other examples of units, I have a huge PDF of my units that would give you an idea of the sizes of things.
https://www.fysh.org/~katie/wargames/downloads/flintloque3/