"most of his paintings were still lives" sounds as though "still" is being used as though they had not stopped being lives, not as though they were of things that were keeping still (iyswim) It seems to be an adverb attached to "were", instead of an adjective attached to life/lives. Perhaps the hyphen between still and life/lives would tie them together as one expression?
The English language gives plenty of opportunity for this kind of discussion.
I remember a proposal a long time ago that the language should be pruned of most of the long words, and their place taken by combinations of short ones - easier for foreigners to learn fewer words, it was suggested. But they had not taken into account that then all those combinations with unlikely meanings would all have to learnt separately, and not mixed up with similar combinations. (put in for promotion instead of apply, put up for sale instead of offer, put out about something instead of annoyed, put down for euthanased, put forward for proposed) I think when learning a foreign language I would rather have one word with one meaning.