Ya, play with it, if the water is just slightly lower than the output level on the skimmer it should work fine in the sump, but like Amphiprion mentioned, it would be better to get the pump for the skimmer as high as possible in the sump to work the best.
Is this the Pro or regular Remora model? The regular one uses a Maxijet 1200 for the pump. I believe the Pro's use a Mag3 for the pump if I am not mistaken, which is a heavier pump and harder to get up high. I think you need to support the pump somehow and not suspend it off the tubing that connects it to the skimmer because the weight may eventually pull the pump off from gravity and vibrations and cause a flood, even with the best clamps that are safe for saltwater, which pretty much limits it to plastic clamps.
As for your sump, I would actually go with something like a 40G, something in 36" flavor IF your 120G is 4' long. If it is 6' then definately go with a 4'/55G sump. You will want the extra room under the stand for other equipment down the road so don't fill the stand with a sump end to end.