Here is an older picture of my main xenia colony. This was taken a few months ago and it is pretty much twice the size of this now and extends a lot further.
It does well in pretty much all conditions.
Lighting:
I have kept under low powered Power compacts, medium amounts of T5 lighting and high amounts of T5 lighting. It did fine in all of them (though now its under about 250w of T5 lighting it grows EXTREMELY quickly).
Flow:
I have kept in all conditions from low flow to pretty much direct flow from a korallia 4. It has done fine in all of them but seems to extend further in lower flow and pulses more.
Another note of water flow: Xenia will grow in the direction of the strongest current. As such it is fairly easy to get it to stay in one place by regularly moving the rock it is on so the current is flowing in the other direction (keeps it moving towards one spot all the time).
General notes:
Xenia once established can grow extremely quickly and it can "move" a large distance in a short period of time (especially in higher flow rates). As such it can become a pest by overgrowing other nearby corals if it is not regularly fragged.
I try to isolate mine to one rock. When it grows to the edge of the rock I frag it back or turn the rock so the flow makes it grow back towards the middle.
One other invasive thing it can do is detach parts of itself which will float around until they either get chewed up by a powerhead or settle in a new location, this can be a pain to control but it only seems to do this in high flow when it is running out of space to grow in (I have about 4 new colonies growing in my tank now which I need to try to remove).
The pulsing forms will not always pulse (though mine does except when there is something wrong with the water, best early warning device ever).
Its an undemanding coral and very pretty to watch (especially the pulsing forms with its waving "hands"). Yes it can be a weed when it gets established but IMO it is controllable. Great little coral for filling in gaps (can also be grow on the back/side walls of the tank) and its fairly easy to get hold of (I have TONS of frags for trade if anyone is interested
).