Is there many differnt types of SD's avalibale? If so is there much of a difference in them such as size or behaviour? Seen the red hook type, they seem cool.

I believe there are several varieties of silver dollar, but I'm certainly not an expert on these although I've got a couple myself.

Yes, there are many!
www.webcityof.com/miff1061.htm

What I understand most of them become the same size and all behave in the same way.
There's some excelent info on them in the tankmates section!
Very cool fish.
Yesterday my 6 SD's teamed up to steal the Vieja's food.
They took the wafer and then they seem to play a game American Football with it!
Very nice addition to an Oscar tank!