to answer a few of your questions:
some people have frag tanks to grow out frags. their main tank isnt crowded w/ the frags, yet they have the ability to trade and sell off the frags. it is a seprate tank that is typically plumbed into the system.
sometimes you can keep it on theplug, sometimes you tank it off the disc/plug. whatever your needs are. thats the beauty of it, your tank can be different then another, but similar in so many ways too! like you said if you glue it to something, you'r kinda...STUCK!! :wink:
as far as what you should do next, my advice would be to watch your frags mature into healthy colonies! i totally understand the space restraints and thats why what im doing myself is watching. occasionally ill see something that i really like and ill want a piece, but for the most part it's just watching my tank, "grow up."
hope this helps