A few crosses between tomato and members of the solanum genus (potato family) have been made. These are usually difficult to very difficult to achieve requiring cellular fusion and embryo rescue to get a growing plant. No natural crosses have been found between tomato and eggplant, potato, pepper, or wild nightshade. The ability of tomato (Lycopersicon) to cross with Solanum indicates that the tomato and potato are closely related to each other, don't expect to find these crosses in your garden though.
What about maintaining seed purity? Rogueing tomatoes.
This is something that most gardeners have a problem doing because the best way is to pull up variant plants. I go through my tomato plants on a regular basis and carefully check for plants that do not match the variety description. Any off-type plants found can be removed, left but don't save seed, or possibly save seed and see if the character breeds true. I have a potato leaf heart shaped tomato that is a result of finding just such an off-type plant. The crucial consideration when rogueing is to take every precaution to maintain seed purity. Crosspollination is another item to consider re maintaining pure breeding lines.
How do I make my own crosses?
Please refer to Keith
Mueller's site for information about crossing tomatoes. Keith
Mueller is far more experienced than I am at this and his writeup is the
best that is available on the web.