Sorry that this is so long.
I took a few steps back and kept switching out bedding and scratch mats. Still showing Allegro and Forte each other through a crack in the door. No hissing between the two anymore. Coda doesn't seem interested, or maybe she's afraid, so she doesn't really go near the door.
Poor Forte is SO BORED. We try to spend time in there with him, but while we're gone he knocks over his water (we came back to a flood on the floor), scatters his dry food, and knocks over his bed and kitty condo. I feel so awful for him. Being isolated is slowly driving him nuts, I think. He's so sweet and affectionate while we're in there, though. I sit on the toilet seat and he jumps onto the counter, puts his front paws on my shoulder, and gives me head-butts and nuzzles, purring. My mom called while I was in there with him and I got into a mini-argument with her, and when I raised my voice and sounded upset, Forte jumped onto my shoulder like a parrot and nuzzled me with his nose and head, purring for me. Such a sweetheart.
Tonight we tried putting Forte in the carrier and letting Allegro and Coda look at him while we fed all of them treats. Coda growled and hissed. Allegro was fine, and just sniffed at him, tail up. Allegro went to explore Forte's room and was just fine. Forte was going nuts in the carrier, though. We thought he was going to hurt himself- he kept scratching at the bars (nail sheaths falling all over the place) and trying to dig his way out of the carrier. He was moaning a little. We felt awful, but happy that Allegro was doing well, so we put Coda in another room and let Forte out to explore the loft area (which is a kitty area and right outside Forte's bathroom). Forte seemed glad to get out, and Allegro seemed curious. Both boys had their tails up and were sniffing each other's tail regions, but both boys seemed nervous and wanting to keep their backsides away from each other. They were doing OK until Forte started batting Allegro with his paw (maybe to play?) and Allegro hissed and batted Forte with HIS paw, claws out. Then they tackled each other, growling and yowling. I had treats all over the room, but they were distracted by each other. I tried playing with them using a teaser toy, and it worked for about 5 minutes (Allegro was calmly laying on his side, watching Forte play and blinking) before Forte stopped trying to pounce on the toy and instead turned his attentions to stalking and pouncing on Allegro. More chasing and fighting and hissing and growling. I ended up picking up Forte and taking him back into his room. Poor little guy. I apologized to all of them for having to go through that.
I know it's only been a week, but I'm very discouraged. Keeping Forte isolated would be fine if he were a cat like Brooke (the foster mom), because she never expressed interest in leaving the room and was just fine (and felt safer) chilling with her toys in the bathroom. It's not enough for Forte; he needs someone to play with, and I desperately wish I could let him out and they could play. I almost wish things had been different and we had gotten Forte first--maybe it would have been easier.
Basically, I know it's all about THEIR schedule, and not OUR schedule...but I don't know how much longer Forte can take being by himself. At the rescue, he was around lots of cats and dogs and people all the time. Here, he has no one, except when Jake or I go in there (we each go in there about twice a day to read and play with him). This is not fair to Forte, and while progress is being made in Allegro's case, progress is very slow. I had no idea Allegro would be this difficult--he had no issues with the foster kittens.
My husband has already said we're not giving up--he's too attached to Forte. And for heaven's sake, I certainly don't want to give Forte back, even though it technically is a foster-to-adopt situation for 3 more weeks. But at what point do we determine that this just isn't good for either Forte OR Coda and Allegro? I've heard of cats that just don't ever get along, even if they stop fighting. If that happens, Forte will have no playmates, and he'd probably be happier back at the rescue. And Coda and Allegro would be happier without the stress of Forte being there.
This is really depressing. I didn't think it would be a cakewalk and I knew it'd take time--I certainly didn't think they'd be grooming each other and cuddling at this point--but I know a lot of people who bring new cats into their households, and no one I know IRL has had this much trouble. I guess I just expected them to be wary of each other at this point, but at least able to be in the same house together without fighting.
We're giving it more time, of course. Wish us luck.