Researching a particular house interior design style can be a stimulating process in itself. It can involve visits to museums, libraries, historical homes and even trips abroad. A book, play or film might prompt interest in a specific era and involvement with a local historical society could provide valuable background information on the interior design styles you eventually settle on.
The words 'authentic' and 'pure' are often bandied about fairly indiscriminately when describing historical styles. But just as perfectly matched colors can result in a rather boring scheme, so, by being fanatical about a particular period or style, you may be in danger of creating an house interior design style that is totally predictable and rather lacking in character. Feel free, then, to be creative and, every now and then, to interject an element of surprise or an item from outside the period perimeters. Colors, too, can be liberally interpreted as they were in the past when paints would have been mixed by individual decorators.
Historically, rooms were very much allowed to evolve over the years rather than all the contents being put in place at one specific time and kept as a shrine to their year of birth. Your own modem home may feature an Art Deco wall light, a dressing table in the style of Mackintosh and a chair inspired by the popular designs of the 1950s. So long as the items have a design affinity, there is no reason why they should not be grouped together.