Provided the stand has the necessary support, sure, you can put it on a rectangular stand.
Note: A 120 gallon aquarium will weigh somewhere around 1200 pounds full. It needs to be FULLY SUPPORTED. Putting it in the middle of a span covered by plywood or whatever will not hold it. Glass or acrylic, you need firm, strong support, directly under the edges of the tank. Sitting your half hex tank in the middle of a rectangular table, without reinforcement, is a recipe for disaster.
My tank, about 140 gallons, has a steel structure, the same size and shape as the tank. On top of that, I have an oversized two layer plywood top, extending 10" past the tank edges in three directions. Stable as it can be... but the edges of the tank are FULLY SUPPORTED. The ply top can't sag under the load.
I'm not sure he means in the middle of the stand. I'm taking it as overhanging off of the stand hence the tipping. I wouldn't trust it without some sort of support for the overhanging
You can get a larger stand that will fit the whole tank and if it doesn't have supports under, you can add them or double up the top with another 3/4"ply.