new kittie

hi everyone, i'm new here and have a quick question.
i have a 12 yo cat who's been spayed since 8mo and now i've brought in a kitten who is 10 mo old and also spayed. (my 14 yo cat died in dec)
it's been 6 days and they are still hissing etc. the new kitty is definitely holding her ground.
my question is whether i can leave them out together while i go to work? the older one has lunged at the kitty but no contact has ever been made. i keep the kitty in my bedroom while i'm gone.
any signs i can look for that it's ok to leave them out together?
thank you in advance for all your input.

Typically the best way to introduce new cats is SLOWLY and carefully. I just successfully introduced a (not for long) unaltered 4 month old kitten to my 17 year old cat. It took about 6-8 weeks. For a couple of weeks, keep them totally separate, with separate food and litter. Starting on week two, switch their beddings on one another periodically so they get used to one another's scents. They will also sit in front of the door and meow at one another. That's OK.

If possible, after the second week, start feeding them on opposite sides of the door so they link eating (which is relaxing) with being next to one another (which can be stressful).

Around week 4, start letting them have supervised time together, maybe an hour or two a day. It's VERY important when you do this to IGNORE the new cat so that he/she will have to go to old cat and get attention from there. Otherwise there will be competition for attention from you, which is bad. After a week or two of increasing-length supervised visits, you can leave them alone together when you're out. There will probably always be spats, hissing, etc but with this method new kitty and old kitty sleep on the same bed, eat next to one another and have yet to hurt each other. They do however, get mad at one another periodically and have hissing fights just as people who live together would. This will decrease over time but it's important that you also let them reach a pecking order and an accepted role/spaces/favorite spots, etc.