<blockquote>- Need to put contact button at the bottom of every K2 article automatically</blockquote>
You should try to find a extension on JED http://extensions.joomla.org/search?q=contact+ , may be you will find a good one to work for you.
Or you can search for good freelancer to code it for you. You can even try joomlancers.com
Need a new preset font size in K2 editor (or could just be <h10> or something like that)
H1,H2,H3…., are html tags using for heading text . Normally, only h1 to h6 are used, H7 => H10 are not usually used.
You try to see the JA Typo in here: http://demo.joomlart.com/extensions15/index.php?option=com_content&view=article&id=140&Itemid=128
You can try JA Typo plugin to make special styles for the k2 item’s content
Thanks