To change default swimwear/pajama:

The sims are categorised by sex, skintone and body type (exept children).
F = Female
M = Male
lgt = Light skin
med = medium skin
drk = dark skin
fit = fit body
fat = fat body
skn = skinny body
chd = child

Replace the default Pajama (all sims). Can't differ from child/adult or body type:
Rename the file to:
Female light: "PajamaFlgt_01.bmp"
Male light : "PajamaMlgt_01.bmp"
Female medium:"PajamaFmed_01.bmp"
Male medium: "PajamaMmed_01.bmp"
Female dark: "PajamaFdrk_01.bmp"
Male dark: "PajamaMdrk_01.bmp"

Replace the default Swimwear (all adult sims):
Rename file to:
Female fit adult-light: "UFFitLgt_01.bmp"
Male fat adult-medium: "UMFatDrk_01.bmp"
Female skinny adult-dark: "UFSknDrk_01.bmp"
And so on, see descriptions above.

Change the default swimwear, child:
Female-light: "UFChdLgt_01.bmp"
Male-dark: "UMChdDrk_01.bmp"
And so on, see descriptions above.

To make a set (sim wearing spesiffic outfit only):
Example:
Outfit is called: "b075FaFitLgt_vonl_outfit.bmp"
Rename:
selected Pajama: "PajamaFlgt_vonl_outfit.bmp"
selected swimwear: "UFFitLgt_vonl_outfit.bmp"
selected formal: "FFFitLgt_vonl_outfit.bmp"

Note- Sims are born with swimwear, it will not change if you change the outfit!