I think it’s fine if they hiss a little. I introduced my cats after keeping them apart for about 2-3 weeks. It was REALLY hard, but my older cat is a bit tempermental, so I had to take my time.
When you do introduce them, what I would do is start by keeping the new cat in the crate. I would give them each a small dish of tuna fish– put the plate inside the crate for the new one, and RIGHT in front the crate for the older one.
Give them plenty of time to each eat the tuna- it might take your older cat a bit of time to go up and eat it, but give them about 15 minutes or so. THEN open the door and let the new one out, but leave the door open so she can hop back in if she gets scared.
Make sure you are there the entire time, and maybe try to comfort the OLDER cat by petting it a bit when they meet. The new one is going to be scared, but the older one will benefit from having you comfort him/her to let him/her know that you still love him/her.
They are going to hiss at each other. Let them do it once or twice, and see what they do. If they start to get more aggressive than just the occasional hiss, then tell them both “NO.” You could also keep a spray bottle on hand filled with water (don’t use an old cleaning one– get a new one that has never had anything other than water) in case they get very aggressive, but only use it as a last resort.
There is definitely going to be some hissing going on at first, because it will take them time to get to know each other, know that the other one isn’t going to hurt them, and figure out their place in the household. See how they do together, but if there is hissing, only leave them out with each other for a little bit of time (maybe 20-30 min) and then separate them. Then, day by day, do these steps again (including the feeding– this helps them associate the other cat with something good!), and leave them together for a little more time each day.
It took me about a month until I’d leave the cats out together while I was gone, all the time. Now they do just fine with each other!