Hi lms1,
If you have an antique bedroom set I would measure your bedframes to make sure they are a “standard” size because some of the antique bedframes are non standard sizes.
If they are a standard size then of course a standard size mattress and foundation would be fine but if they’re not then many manufacturers or retailers would be able to supply a non standard size as well but I would confirm this before you make a purchase.
There is also more about the pros and cons and some feedback about split mattresses in post #2 here and the posts it links to that may be helpful.
As you probably know from your reading here … one of the advantages of a “one choice fits all” mattress where there is only one firmness option is that it is very simple way to choose a mattress without spending time testing different mattresses but you will generally find that it will be the “best” match for a relatively small percentage of the population, a “good” match for a larger percentage, and an “OK” match for a larger percentage yet. The only way to know which group you fit in will be based on your actual sleeping experience but for those where it’s not a “good enough” match then Tuft & Needle has a great return policy that lets you start all over again with choosing another mattress with little risk outside of the time you spent trying the mattress or returning it.
Assuming you decide to pull the trigger (or have already) … congratulations on your new mattress 
I’m looking forward to your feedback when you receive it and have had the chance to sleep on it for a bit.
Phoenix