We've got four - we went for three, but the breeder said that mixed sex groups of odd numbers would fight.
It is ok to keep one, as long as you spend time with it. They are sociable, flock birds, so if you become it's flock it will love you forevah.
Pairs or more will never be as tame as a singleton, and certainly will never talk. But, ours are perch trained and happy to sit on a finger if it's brandishing a dandelion leaf or millet. They live in a giant cage in the living room - though, I think next year we're going to build and aviary and keep quails in the bottom and breed them...
well, it'll be better than me having another baby - baby budgies, too cute.
Anyhoo, slipper -if the stuck on poo is sort of solid and not green and runny with sore skin round it's vent, and the budgie is happy and eating and active - then I'd figure it was probalby just a fluffy one. Try a budgie bath or misting him with water and see if that encourages some proper preening.
In fact, I might try that method on my sons who often also have poo ishooos.