It really depends on how long the cat has to stay in the crate. If for more than a day or days, I found a
large size dog crate is the best size, as you can get food, water & litter box in without it being too crowded. My hubby made a carpet-covered shelf to fit in it. A cat playpen would work as they take up less floor space since there's more vertical height and often have more than one shelf, but they don't give as much walking-around room as a dog crate. So your choice.
As far as adding more cats in your apt., "fat chance" is right! It would likely stimulate more frequent peeing.
Altho my cats are related (same dam, different sires), they didn't grow up together, tho they seemed to sense they were kin and by the second day they cuddled together.
I'm glad you feel I've helped......
Take care.