I have a Montegue and use an Oasis with a couple of pounds of HCM beads throughout. I keep mine in my finished basement and find the RH is very stable in the summer, but as the winter temps roll in and the furnace runs more, I have to fill my Oasis every couple of weeks.
I would suggest doing a flashlight test once you have it set up. I found a 1/8 - 1/4 inch seam along the top of the unit which I sealed by jambing weather stripping into the crack. I also weather stripped around the door as light was passing through there as well. I used a weather stripping that compressed more for the doors as I needed to make sure the doors stayed shut.
I think a Hydra and a couple pounds of beads would do the trick. As for seasoning it with just the Hydra, I think you'd be better off leaving some sponges on plates in there as the SM Hydra may need a few fillings to season it properly.