You can change what races appear in the guilds and slave market by editing just one file :
/assets/data/worlddata.gd
Open it in any text editor (it's just plain text). See line 9, where it says
races = [['Human', 50], ['halfbreeds', 15], ['Elf', 3]]
That tells you what races will appear in guilds. You can change the frequency by simply editing the number, or even add new races; for example :
races = [['Human', 30], ['halfbreeds', 25], ['Elf', 8],['Centaur', 200]]
would make centaurs appear more often than any other race. You can add any of the races to the list :
Human,Elf,DarkElf,Drow,Orc,Goblin,Kobold,Gnome,Dwarf,Fairy,Dryad,Demon,Seraph,Dragonkin,Centaur,Taurus,Harpy,Slime,Lamia,Archnam,Scylla,Nereid
as well as Beastkin# and Halfkin# for Cat, Wolf, Fox, Bunny and Tanuki. You can even add your own custom races.
If you just want random races to appear sometimes, you can add ['rare', #] or ['monster',#], replacing the # with however common you want them to be.
Monster races are : 'Lamia','Scylla','Centaur','Nereid','Arachna','Slime','Harpy','Taurus','Dragonkin'
Rare races are :'DarkElf','Drow','Goblin','Gnome','Kobold','Dwarf','Seraph','Demon'
However, this needs to be done before you start a new game, as these settings are saved in your save game file. You can edit the save game to change the races, but it's more complicated.