Here is my understanding - although I have not been in a clan for a long time and so I'm happy for other players to correct me.
A single character can only be in one clan at a time (whatever rank they happen to be). However, the person behind the character can create an alt and join or create another clan.
For example (using your forum name):
original is in clan 'one'
original2 is in clan 'two'
As for your question about building, here are the game rules.
viewtopic.php?f=25&t=2560
Within them is rule 14 about spam building when in clans. You will also find links (near the section in yellow about GM Inbox) to the game manual and the game FAQs, which contain further information about clans.
I hope this helps and that others can add to this to answer your more specific questions.