Opalka, Serrano, Cilantro, Vidalia, Jalapeno, some type of sweet bell. I was also thinking corn, but one stalk won't give you all that much compared to the other plants listed. You could also do some type of bean although that doesn't match your typical salsa.
Jalapeno, Serrano, Orange Hab, garlic, cilantro, shallot. But doesn't garlic grow from a bulb? It is essential for salsa, but I don't know how practical it would be for a salsa pack. Maybe substitute a NuMex Barker for that one.
roma tomatoes, white onions, serrano's, orange habs, cilantro, orange or yellow bell.
thats all you need to grow to make good salsa in its self. though lime juice, black beans, corn, garlic are all great to add in salsa but you can buy those items.
The fresh salsa I make every Sunday for the coming week is just Garlic, Onion (usually use a sweet and a white onion), Serranos, Jalapenos, and Tomatoes. If I could only grow 6 plants, I would ditch one of the onions and use the extra spot for a scorpion plant to have the option to add extra heat when the salsa is just for me.
Given my constraint - having to be able to grow the pants in a six-pack nursery flat, and that I want to sell them, I would have to pick:
Jalapeno Pepper - everyone knows them
A Roma or paste type of tomato as well as Green Sausage. Very prolific plant, next to no seeds or juice.
A tomatillo, but these things are a real PITA to grow as seedlings.
Cilantro - a necessity.
An onion - Walla Walla or Yellow Spanish.
A bell pepper - so much flavor I cannot leave them out.
My salsa includes jalapenos, vidalia onion, cilantro, roma tomatoes, orange habs, garlic, salt, pepper, and olive oil. So I guess my sixth plant would be an olive tree!