Laying with incline.

Baby spits up constantly. So we've been told to lay her down at an incline so it helps the milk stay down. So we bought one of those fisher price rock n plays that are slightly angled. She's been sleeping in that every night for 9 weeks now. My question, am I shooting myself in the foot? Will it be impossible to transition to a flat surfaced crib? How long is it okay to let them sleep in those through the nights? The other thing is in this sleeper she sleeps for a 6 hour stretch through the night! When we tried the crib for naps she lasts 1 hour max and hates it... 😩 any tips?