First of all, welcome to the cichlid hobby.
Typically this shaking and vibrating is aggressive behavior though it can also be a sign of mating. Just because there is no attack carried out doesn't mean that it isn't aggression, in fact most of the dominance displays between fish don't result in any physical interaction, its mostly just posturing to the other fish. What it means is that your socofoli is showing the other fish how big, strong, and dominant it is. The color of a lot of cichlid changes with mood, personally I have a few fish that look best when they are really angry at other fish.
As far as using this behavior to sex the socofoli, the shaking doesn't necessarily make it a male.
2.5-2 inches is a large enough size for those fish to be breeding, though I personally wouldn't worry about hybridization quite yet. Given that your not sure on the ID of your fish it is still hard to judge the risk of hybridization. Also because you mentioned you purchased "mixed african cichlids" you should know that fish from those sorts of tanks are all too often hybrids.
If you have any more questions feel free to make more posts, this forum is an excellent tool because there are so many members with years of experience in keeping fish. For the fish that your aren't sure about the ID there is a section of the forum where people can help identify them.
Typically this shaking and vibrating is aggressive behavior though it can also be a sign of mating. Just because there is no attack carried out doesn't mean that it isn't aggression, in fact most of the dominance displays between fish don't result in any physical interaction, its mostly just posturing to the other fish. What it means is that your socofoli is showing the other fish how big, strong, and dominant it is. The color of a lot of cichlid changes with mood, personally I have a few fish that look best when they are really angry at other fish.
As far as using this behavior to sex the socofoli, the shaking doesn't necessarily make it a male.
2.5-2 inches is a large enough size for those fish to be breeding, though I personally wouldn't worry about hybridization quite yet. Given that your not sure on the ID of your fish it is still hard to judge the risk of hybridization. Also because you mentioned you purchased "mixed african cichlids" you should know that fish from those sorts of tanks are all too often hybrids.
If you have any more questions feel free to make more posts, this forum is an excellent tool because there are so many members with years of experience in keeping fish. For the fish that your aren't sure about the ID there is a section of the forum where people can help identify them.