I have never tried to grow casuarina inside. We only grow these outside in natural conditions so I am not sure how they will grow for you.
Casuarina is not really like any other trees. The leaves are tiny. You can only see them with a magnifying glass. The green part is actually the stem of the tree. Pruning will gradually make the stems shorter and build up some good branches but the first thing is to grow a trunk.
Normally we let the tree grow wild for a few years until the trunk is thick and has a good shape. Then we start to select some good branches then grow some twigs and leaves on those branches. it will take quite a few years.
Casuarina is not like a pine. casuarina 'needles' are actually the stem of the tree. They are divided up into joints where the tiny leaves are. To limit the length of those stems you can cut anywhere or you can pull the stem apart. The stem will break at a joint. Just keep trimming or pulling whenever the stems get too long.
Language may be an issue. I said when the stems grow about 4-5 cm long you can trim them back to make shorter - cut back to say 2cm to make the tree look neat. Trim shoots whenever they get long and look untidy. This is the way to increase the miniature look of casuarina.
This thread has some pictures and explanation of how these trees really grow -
viewtopic.php?f=56&t=24470&hilit=+casuarina+stem