Installing background in running tank

Hi everyone - I'm still here, reading and learning (though often
waaaay behind!)

My Rio 125 has been running for about three months. The current
inhabitants are six mollies, four danios and two neon tetras. Have
been treating ich since last Saturday, though all looking good now.

I want to add a background, more gravel, live plants and some more
fish (starting with clown loaches, Gill ;-)). My main question now is
about the background. Can I install a background in an aquarium that
contains water? I'm planning on the Juwel STR (two pieces of 450). I
thought that if I lowered the water to between 1/3 and 1/2 capacity,
I could slide the background in on an angle and silicone the bit
above the water. Is this do-able or am I going to have to empty the
tank and move the fish?

How long after aplying silicone before I can re-fill the tank? And
will my filter survive for that length of time or do I need to remove
it and bag it?

(Planning to buy the background today, so quickish answers would be
great! :-) )

--
FishNoob