I just got mine a week ago... so I can't answer some of your questions, but I can tell you the basic stuff on them.
Clown Loaches can grow to be a foot long, but they grow
very slowly. When they're small, they can be kept in smaller tanks (I'd say 30g minimum), but will need to be put in a larger tank as they grow bigger. They also need to be in groups of at least three.
I personally have mine in my 90 gallon, and was told by the guy at Big Al's that that'll be large enough for them for at least 5 years. (They're at about 2-2.5 inches right)
And from what I've read, their lifespan can be up to 50 years
Hope that helps you some