Ok FIP is a very difficult subject - as you know from Mr Cuddles it can only really be 100% confirmed at autopsy which is awful and I am very sorry for your loss
Spaz is a different kettle of fish - why do the vets think he has it aswell? what are his symptoms or is it because Mr Cuddles had it they are 'assuming' ? Or have they done a test ie Corona count? As really a count is very difficult to interpret and can't be relied upon alone.
If Spaz DOES have FIP then there is no cure, I hate saying that, and if so I really wouldn't get him a playmate as cats are very ill with FIP and will eventually go downhill and pass. Bringing a new kitten into that situation would be very difficult and not the right thing to do imo.
Vets tend to look at occurrences with FIP also in colonies, families, environments etc if there is more then one case occurring - so again if Spaz had it aswell then there would (imo) be some sort of genetic link. It is actually quite rare for FIP to mutate from Corona and just because a cat has a count does not mean it will go any further then that. Most cats and kittens will have been exposed to the virus, and many will be carrying it - esp in multi-cat household.
If I were in your situation I would wait until Spaz is around 2years old before I looked to introduce a playmate, and that he is of good health - no ill signs.
FIP is stress induced - bringing a kitten or another cat into the mix right now would not be wise imo.
There is lots of information you can seek - please take a look at this website
http://www.dr-addie.com/Again I am so so sorry for your loss x