You likely will be fine at 55 minutes. I've had a handful of similar connections and always made them. Immigration used to have people who would pull you out of line if you had a short connection if line was busy. But I don't know if they are still there. My most recent connection was 50 minutes OSL-EDI and had to clear immigration. But line was so short they didn't need a priority line. Only slight worry would be how far apart the gates are. I don't recall the walk being particularly bad from EU to USA on Delta or KLM. But my recent KLM connection involved an almost 20 minute fast walk as our gate was at end of terminal.

3.5 hours at Schipol isn't too bad. I actually prefer that timing as it gives a moment to stretch legs and grab bite from lounge. Rushing from a long flight to get on another plane isn't ideal. 7 hours is too long to be in airport but not really long enough to go to city and actually do anything. You could probably get off train, walk around near the station, and then go back.