The pot is large and adequate for that size of plant.

When removing the plant last year, did you take note of how many tomato roots made it to the sides and bottom of the pot?

If the tomato roots reached all areas of the pot, that would indicate than any additional plants would compete with the tomato. If it produced more tomatoes than you need, maybe you can tolerate this root competition.

Or with multiple plants, you could just fertilize and water a little more.

What I took out was a surprisingly small root ball. Maybe they rotted, maybe they were small to begin with, maybe they were small due to possible over watering. At any rate, they would scarcely fill a gallon milk jug.

Aside from root competition, I was wondering about compatibility. I am told that some plants just are not good neighbors.

You can plant those small herbs in there, they won't harm the tomato, maybe the tomato will keep them from getting as big but that is not a big deal if it's a choice between a smaller herb and no herbs.

Basil and tomatoes grow well together. The basil is supposed to make the tomatoes taste better. I’m trying the combo this year to see if it’s true.

I will do so too. I like to put whole branches of Basil directly into the water to cook pasta. It comes out in one catch and adds flavor.

My personal preference would be to dig a hole where that pot sits and plant it in the ground with the basil next to it (my thumb turns brown when it comes to containers for veggies).