An ATPL is a license. On that license you have have varies ratings, eg 737, light a/c are counted as SEP (single engine piston) So to fly a light a/c all you need to do is keep your SEP rating current.
And NO, Hours in light a/c do not count towards the rolling 12 month 900 hour limit. only a/c over 5.7 tones are. (A Cessna 152 is about 1 tone)