that still seems like a small tank as BTAs can get to be over 2' across.
Though BTA's can get that large, they don't seem to regularly hit that large of a size in our tanks before they split. I've only seen two that large, and both were in very large, established systems whose owners were a bit OCD with keeping parameters exactly even at all times. Most BTA's seem to split before they reach that size. Additionally, a 28ga. tank IMO is not too small for a BTA. I know of someone who keeps a haddoni in a tank about the same size. The key is to be responsible, and if/when the nem gets too large for your tank - get rid of it for something smaller. It's going to be harder keeping water parameters in check with that size tank, but it's not too small.
It will move for the sake of moving even if it has a good spot.
I would respectfully disagree with this statement (assuming we're talking about bubble-tip anemones). It takes energy (and is risky to its health) to frequently move around. If your BTA is moving around it is for a reason. Is there adequate lighting, what are the age of bulbs, what is the flow like, is there an adequate hole or crevice to secure its foot into? What are the water parameters? are they stable or has there been a swing lately? Assuming all of these questions can be answered beneficially, the BTA is very unlikely to just move "for the sake of moving". If there is mechanical damage, or splits, or the questions above are answered in a detrimental fashion, then there is a likelihood that the BTA will, in fact, move. The problem is that sometimes we may not know what changed, or what is wrong and attribute the moving to "it wanting to" rather than something we may have overlooked.
jesterman22 - Every time anything is added/subtracted from the tank (rock, sand, even new fish) the tank will undergo a small mini-cycle. So when setting up a new tank - even when using old rocks - you will encounter a cycle. With using old rocks the cycle will usually be less than your original, but definitely don't discount the possibility of a high ammonia spike that could kill delicate livestock if added too soon.