It is useful to have somewhere to be able to put the foster cat apart from your own. Most foster cats - even if you do let them mingle after an initial quarantine period need to be able to be contained at some point - eg after ops, when on medication, to allow you cat free access to a cat flap.
Also I think cats appreciate routine.
keeping them apart from your own also helps create an emotional distance when you have to home them.
Also, you own cat(s) will come to resent all the comings and goings of strange cats into their own home territory so you may get more stress issues - ie spraying. So even though a pen in your living room can be a minimum requirement it is not ideal.
If you do give it a try, make sure you only take an easy cat to start with - not all cats are easy going. And please stick it out long enough to home the cat.
We have had people come on board as fosterers only to last a week - and thats after a protracted time for them to be able to have second thoughts.
We put effort and expense into setting up new fosterers so this is then wasted.
Also because foster space is so short, it usually means we have taken in an extra ca that we would not normally have space for and so are then stuck with having to decide where the cat can be moved for. Its also not good for the poor cat to be shoved around more than necessary.
Having said all that, please do not let an outside chance of it not working put you off, but just accept to stick it out for that first cat.
