Frankie, since it seems you plan on having tangs then yes a 6' tank would be best for them.
Yes, some people have tangs in smaller tanks. But they are really small, and they are prepared to re-home them when they get larger.
Your first fish should be fish that you truly would like to keep.

Start with the gentler disposition fish first, leave the aggressive one for last. Reason for that is that if you add the aggressive one first you will have a very difficult time adding any other fish.
CUC = cleanup crew such as snails & crabs which are considered invertebrates. But are tougher then the other invertebrates like anemone, shrimps,so on & so on.

The later invertebrates should be added when your tank is cycled & stable (meaning no swings in parameters). It is said most saltwater tanks can take upwards of a year to fully mature, but you do not have to wait that long to introduce most invertebrates.
I would suggest to you to research the types of fish you would really like to keep in your glass/acrylic box.

Research their requirements, adult sizes, and temperaments. I would also suggest you stay away from those classified as difficult to keep until you yourself are more experienced in the saltwater hobby.
Also, once you do your research, and have a list of fish which you would like to keep you can post it on MR so that folks here can give you their opinions on them.

Damsels can, and are very nasty territorial little fish, and once in your system you will have a horrid time getting them out.
Have a wonderful reefing experience! I love that you are taking your time to learn all that you can before jumping in.
