Can I use these word-combinations to classify different paintings of an artist?
Can I say "nautical theme in art", "natural themes", "seascapes", "still lifes", "architectural landscapes"? Do they suit for categories of paintings?

Question

Answer

Your question should be:  Are they suitable (adjective) for categories of paintings?  (or for painting categories)

Categories need to be all nouns or all adjectives for good parallelism: (let's use all adjectives)(adjectives are singular in form)  
- Nautical, Natural, Seascape, Still-life, Architectural