I am new to gardening but I'm starting off with heirlom seeds. I wanted to plant multiple varieties of tomatoes and peppers. Can I do this or would it affect the plants by cross-pollination? If it is an issue, could I just plant them on opposite sides of the garden? It's not a terribly large garden, 20x10 or so. Or does it really matter?
Unless you are saving the seed then it doesn't matter. Tomatoes are self fertile for the most part, so while you may get some crossing many people are saving seeds from unbagged tomatoes with out a problem. Peppers tend to cross more, but not badly.
If you're not collecting seeds, then I wouldn't worry about it. just remember to keep them spaced properly when their seedlings are about three inches. Most tomatoes are fairly big plants and take up a lot of room, and if you plan using that same little plot next year, be sure to rotate your crops. Tomatoes are heavy feeders and drain the soil of vital nutrients and protiens most plants need. Any type of beans or peas would be a good plant to use in its stead.