My 70+30 gal sits on hardwood floor also. I don't use any protector under the stand. Properly finished hardwood can stand a bit of water (I wet mop mine regularly), so as long as you mop up any spill promptly, it should be fine.
The problem comes when water gets trapped between the stand & floor, and the dampness will ruin the finish. Personally, I think that putting plastic or other "proctector" between the stand & floor is counter-productive. Any water that got under the plastic will sit there & not evaporate & ruin the finish is short order. I custom build my stand so there are only 6 contact points between the stand & floor, and a 3" gap underneath, so any spills will dry quickly. So far, I've not have any problems (2+ years).
Now, if the cabinet is completely closed at the bottom, you might have problem once water gets under it. Putting plastic under the stand is not a solution, unless you use something big enough to cover a 2-3 foot wide area all around the stand, so that no spill will get under the plastic. But that would look butt ugly!
Raising the stand to ensure air circulation underneath is the most effective solution. But depending on the stand's construction, may be impossible or not structurally sound. A raised stand really should be designed from the ground up. It may be possible to cut ventilation holes/slots from the back or bottom of the stand .... but make sure you don't cut any structural elements!
Failing that, I would just use the stand as is, and be careful to not get water under it. You might end up having to refinish the floor if you decide to move the tank after a few years. <But then, my floor need a refinish every 5 or so years in the high traffic areas anyway!>
As for levelling, use softwood shims. Cedar shingles work well. If you drive 2 in from opposite side of a leg, you end up with a level, well supported shim. <As opposed to the typical slanted shim driven in singly, the shim is slanted so there is actually a very small area of contact for load distributuion.> For looks, you will want to trim off the excess. This is trickly to do over hardwood without scratching the floor. You can try cutting carefully with a dovetail saw (slide a sheet of hard plastic under the cut line to protect the floor while cutting) .... but safer to mark, remover the shim, cut & reinstall (more work .... you'll have to drain the tank to lift the stand to get at the shims .... )