Hi Tiger,
I've got 9 persians and I definitely wouldnt inflict a Bengal/Siamese cross kitten on them LOL! Lovely though both Bengals and Siamese are! I have 3 moggies as well, and I find that the Persians have more of an affinity with each other, than with the moggies, although in general they muddle along together, and one of my persians absolutely loves one of my moggies, although the feeling isnt always mutual!
I'd stick with your original idea of a similar breed and do the introductions very carefully and gradually to avoid upsetting your persian, i.e. separate room for the newcomer, let them get used to each others smellls for a few weeks before any face to face meetings, supervise all initial meetings, feliway plug ins etc. Don't know about age, all of the ones I've introduced over the past 6 yrs or so have been oldies, but I wonder if a kitten MIGHT be a better option in your case, as perhaps your persian won't see the kitten as so much of a challenge as she would an older cat.