I used a plastic bottle to drink from (pareve drink, room temperature) during a fleishig meal. Later, after rinsing out the bottle, I poured in a lukewarm/hot coffee with milk. It’s a disposable bottle, so my only question is can I drink the coffee? If yes, should I avoid doing this ever again?
Since the bottle is pareve and you washed it out, therefore the water is still pareve, and pouring it into the coffee is fine.
Although halachically it is fine, it isn’t a good habit to mix things used for fleishig to milchig.