What's more serious, a hiss or a wail?

Since my boyfriend and I may have to get his parents to watch our cats when we go on an upcoming trip, when we were taking care of their indoor/outdoor 12 year old female cat, we carefully introduced one of our 7 month indoor females to her, on their porch on a harness and leash, which she is used to. The 12 year old cat was not on a leash of course because she goes outside all the time. The 7 month old hissed, and the 12 year old made this Rrrrrrrrrreaaaoooooeoeoeoeeoeoooowwwwwwwwrrrrrrrrrrrr wailing noise kind of like cats make when they're fighting, but longer and more stable with vibrato. Even the tone sounded like if it was a human sentence it would be a warning.

What's the difference in what the cats were trying to communicate? Which one was more afraid/angry?

They are both afraid. The one who is hissing is saying, "I'm warning you. Don't come any closer." The one who is wailing is saying, "You wanna fight? Do you, do you? I'll tear you up. Just try it."

So I guess the wailing is the more serious message? Before we introduced them they knew another cat was around and we let both of them smell something with the other's scent and we played with them both. Do you think these cats will ever get along? The little one was the first to make a sound (the hiss.) Probably should have let her adjust to the porch like we let her adjust to other areas there, which is what we wanted to do, but the older, outdoor cat was already on her way to investigate. The outdoor cat is used to defending her territory from other cats so making new friends might not be her thing, what do y'all think?

Yes, the wailing is the more serious vocalization. It's doubtful that the outdoor cat is going to allow other cats on her territory. She's guarded it for too long.  It would be better if your cats were restricted to one room, like an extra bedroom, while you are gone, if it's no longer than a week or so.

Yeah, that makes sense, it will be less than a week so that will work. We hoped that they would all get along, since we have read about older cats teaching young ones the kitty ways, but the older cat has been independent all her life so maybe it's not meant to be.

Oh I meant to say the older one was indoor/outdoor*

That's the best solution since they won't be living together permanently.

I agree that keeping them separate is the way to go in your situation.  Cat introductions can take months; so if they are only to be together for a week its more likely just going to stress them (and you) out.