Self- or insect-pollinated, pepper varieties of the same species will cross-pollinate. There is no crossing between varieties of different species, however. You can safely grow one hot or sweet pepper (C. annuum) and one Tabasco pepper (C. frutescens) without danger of their crossing.
Peppers within the same species can be safely isolated by 500 feet of separation, or they can be caged since the plants are not overly large. Allow peppers to ripen and dry fully on the plants before harvesting the pods. Wash your hands thoroughly with soapy water after harvesting hot pepper seeds, since the residues will burn eyes and lips for some time after contact!
Pepper seeds will keep for 2 or 3 years if properly stored.