Beginner's Advice Please

[Miss Wiggle]

One other question, if I buy the tetras in small groups would each group eventually join together into one single shoal?

yes

they'll socialise together instantly, you won't end up with two distinct groups.

remember though schooling behaviour is a sign of fear, fish only group together in a big school when they are frightened of something, so they're not gonna be doing it all the time in your tank. They'll still hang around together though.