Matterhorn...On this ride it always seems the SR line is ridiculously short - as in less than five minutes.
I've done this one a lot too and have had much more mixed results. I've been there when they closed the line due to length (if the line reaches the turnstile, they'll shut it down until it clear out a bit). In general it's faster than the standby line, but I've waited anywhere form 2 minutes to 30 minutes. It's also not always a function of how many folks are in front of you. As others have said, it all depends on the CM, how hard they're going to look in the standby line and the distribution of the standby line.
I've been sent to row 3 occasionally for the Matterhorn. But I do think you're more likely to end up in the back of either of two cars than the front or middle (unless someone asked to be in a specific row and they're slotting you in).Aside from the time my friend and I were thrown on together, I've never seen an SR ride anywhere but the back row. I suppose a guest who's never wedged in there might ask to switch, but not likely.
I agree as well. Seems like a natural ride to have one (although perhaps figuring out how to get folks up to the loading area is the problem) because of the ride seating. It's not a bench or even a car where a stranger next to you might be a bit weird.I wish they had SR for Star Tours. They always tell me they love parties of 1 and 3 when I go through there.
On Indy the CMs will actually point out that the single ride slip says you will be separated. The loading system there is terrible, the cars often leave with empty seats because the CMs can't slot people in as fast as the ride loads. (It may also be a weight distribution thing on the empty seats, but since the cars pull forward before half the riders have seat belts on, I tend to think it's speed.) Then in the rush the far side platform almost never pulls from the SR holding area, and can be so many SRs waiting they'll suspending handing out passes. Forget fixing the eyes of Mara or the fires, they need to fix the way the ride loads. Anyway, I've been an SR on that ride with an empty seat next to me while other SRs wait, but I've rarely seen 2 be allowed in the same car.
On Matterhorn, I did end up with a friend once in the same car, but basically a party of 10 from a tour barreled through the exit, pushed aside a the GAC party waiting with a wheelchair and immediately started hassling the CM, so the CM cleared out the SR queue to have somewhere to put them. Another time my sister and I were split, then the kid behind me ended up in the other car paired with mine. But for the 2 times that's happened, I'd say there's been 4 times where I've gotten off the ride to the person behind me still waiting to ride.
On Splash I have never seen 2 SRs ride together.
On Goofy's it's pretty unlikely because there would have to be 2 single riders that went through the line together for there to be 2 empty seats in to vehicle.
On Screamin you might be on the same train, but like Indy and Matterhorn the CMs would rather pull a party of 2 from the middle of the line than go to the SR line to fill spots.
I haven't done Soarin enough to know your chances, it's a horrible ride for SR. They hold you around the corner where the ride loaders can't see you, and by the time they call you over the ride may be loaded and you have to go back. I didn't feel like it saved any time the times I've done it, so I stopped. And I've never done GRR, so no advice there.
RSR is probably the best chance that 2 SRs have to ride together, though it's just as likely to be the stranger in front of you instead of your friend behind you. At the very least there are usually 2-3 SRs in each batch of 4 cars, so you're at least likely to be on the ride at the same time or in quick succession.

My weirdest SR story was on RSR. It didn't happen to me, but there was a group of four in the party ahead of me. The CM told them to go to "two to row 5 and two to row 6" (I don't remember the actual row numbers). The mother asked if they could ride 3 and 1 so the three teenagers could ride together in the front row. The CM said no because they were adding two single riders to their car. So they lined up as told (two in row 5 and two in row 6), but they must have talked with the SRs because when they got into the cars they re-distributed (3 in the front and the 1 plus the 2 single rides in the back). That seemed so weird to me that the CM wouldn't allow the group to distribute themselves how they wanted.
I agree as well. Seems like a natural ride to have one (although perhaps figuring out how to get folks up to the loading area is the problem) because of the ride seating. It's not a bench or even a car where a stranger next to you might be a bit weird.
I wonder what they would do if someone asked to not have a single rider with them...I suppose they would honor it (at least I hope they would). I can imagine on TSMM where it wouldn't go very well. I've done single rider thrill ride where you share a car with someone, but they've always had distinct seats and safety gear. It does feel a little awkward, I'll admit. I suspect that's why things like Big Thunder won't have a SRL...bench seating and a shared safety bar. Yikes!