How to add content inbetween node fields on node creation?
I guess this inbetween content is static or else you would have it as a field as well. So, I think you should just edit the template file of this node and add the fields and whatever other info you want between them directly on the file.

It would look sth like:

&lt;?php print render($content['field_firstfield']);?&gt; // first custom field
&lt;div&gt;
whatever you want inbetween
&lt;/div&gt;
&lt;?php print render($content['field_firstfield']); ?&gt; // second custom field
&lt;?php print render($content['body']);?&gt; // all the rest content