Sorry, I have only just seen this.
Ruminants need forage from a week old to help the rumen develop. Some people prefer them to only have their bedding straw until they are a little older, so if you go this way, make sure they have some clean bedding straw fresh every day.
If you give them hay at a very young age, it needs to be lovely soft fresh hay, not dry and scratchy old stems.
Start to give them a bit of some scratchier stuff too a bit later on, maybe from a month old, as it helps with rumen and gut development. (But not exclusively scratchy stuff, you want them mainly eating lovely fresh stems.)
You might see a little blood in the poop for a day or two as they start to process the stems, particularly any scratchier ones. A little for a day or two is not a concern, but a lot, or going on for a longer period of time, or of course if coupled with other health indicators, would be.
Bloat is always a concern with any young ruminants but especially bottle-reared ones. Basically, don’t overfeed any non-milk foods, do not give too much milk at one time, and for safety, leave a gap in time between giving non-milk foods and milk. Lots of good threads on the subject here - have a browse.