If you allow an heirloom tomato like golden peach to be pollinated by pollen...
If you allow an heirloom tomato like golden peach to be pollinated by pollen from another golden peach heirloom tomato whether by bees or by taking pollen from golden peach plant #1 and pollinating golden peach plant #2 have you created a hybrid? I am sure bees open pollinate from one heirloom to another all the time!
Benton County Missouri
Since tomatoes are naturally self-pollinated, they (genetically) are consider to be homozygous. Crossing two homozygous lines results in the formation of a F1 hybrid. Therefore, you are correct in your assumption that cross-pollinating two heirloom tomatoes will result in a hybrid.