Hmmm........some type of swiss chard?

Doesn't look like chard to me... but break off a leaf and smell the stem. Chard has a beety smell.

red stemmed sorrel.

Pretty sure that's Rhubarb of some description. A Rheum. Perhaps one of the species, perhaps a seedling. Pull a stalk with a slight twist. It should come away with a white scoop shaped base. And it will smell like rhubarb.

I pulled a stalk with a twist, and it does not have a beet-like smell. The stalk does taste like it's got some acidity like rhubarb would- it just doesn't look like my other rhubarb plant.

It's hard to say what it is. I suppose when in doubt, it's safest simply not to eat it.

Can you post a picture of the stalk you pulled? And what is the large leaf off to the right? Is it rhubarb?

Here you are floral_uk. You're absolutely right, the larger leaf in the top right of the first picture is a rhubarb leaf.

I really do think that the little plant is a Rhubarb. Look at the vein pattern - just the same as on your big plant, I bet. Pull a stem from the large plant and compare smell, texture, and how it is attached at the base. Is it possible your large plant set seed? Seed grown plants can be very variable.

I can tell you for certain it isn't sorrel or chard.