I have a trio that goes out under two names, one on the country circuit, the other doing social clubs.
The easiest one to programme is the social club outfit: a song will make to into the set if it has been a top 10 hit and you can dance to it. At least half the set consists of number 1 hits.
It's much harder to please a country audience, not least because some like traditional country, some want to line dance to what is basically rock in a Stetson, and there will always be someone who requests a song you've never heard of. My approach is to work on about 50% 'bankers', I.e. the sort of songs that even non-fans will recognise, and try and tailor the sets to the members' preferences. I have long since given up trying to educate audiences to great songs that have never been hits.
In both cases I limit slow numbers to 1 in 5 at most, and avoid grouping too many mid-tempo numbers together.