The LC in the hospital told me I had flat nipples too and her advice was to try feeding with nipple shield first, then express then use the expressed milk to top her up. Sounds great in theory but that took 1.5 - 2 hours per feed, every feed. So I wouldn't recommend that course of action!

Has her milk come in yet? It could be that the flat nipple issue is being exacerbated by the shape of her breast if it's getting hard/fullish just before her milk comes in.

So yes, I second the try to express a little before attachment and if not, use the nipple shield in the short-term if the attachment seems better with that. Or put it another way, do whatever works and don't worry too much about whether there will be difficulties getting rid of the shield down the track. Cross that bridge later.