A couple of general tips for multipart models:
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If 3D printed parts need to fit together, try to align the critical mating surfaces on the same axis when printing. This will minimize artifacts caused by differences in expansion, contraction, and imprecision.
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If 3D printed parts need to mate together as perfectly as possible, it can be a good idea to print one or both parts without tolerances and match them together with post-processing (sanding).
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If mating together 3D printed and non-3D printed parts, often around 0.5mm is recommended for tolerances but, you'll get the best for by doing test prints and evaluating them (different printers and filaments can have a profound effect).