So my sister believes that I should get a crib with a hard crib mattress for LO, because soft beds can be a lead to SIDs. I was planning on using a cradle or bassinet when the baby is really, really young. I think those things go until 3 months. Then I was just planning on cosleeping/bedsharing.
I already have access to a hand me down crib, so I guess it wouldn't be that much of an issue to get a new mattress for it, but it wouldn't be my first choice. Then again, if sleeping on a futon would be harmful to LO then I guess I wouldn't have a choice anyways. This is the first I've heard such thing.
Re: cosleeping and SIDs
SIDS =/= suffocation although so often the term SIDS is used when a baby suffocates. Technically speaking, SIDS is when there is no reason to be found for the death.
But things like loose bedding, soft mattresses like pillow tops, lots of pillows, etc. can all be a suffocation risk so in that regard your sister is correct. If you intend to share a bed with your LO it's so important to take precautions for safety.
I don't believe futons are considered safe for LOs to sleep on as newborns. Also, make sure the used crib you're considering hasn't been recalled. After all, using an unsafe crib could be a bigger safety risk than sharing a bed.
If you plan to bedshare, google safe bedsharing and you'll find lots of online tips of things to do/things to avoid. We have bedshared with our LO since he was around 6-8wks (I can't remember exactly) and have really enjoyed the experience.
cosleeping and bedsharing are not the same.
There are plenty of safe ways to bedshare, as referenced above. I think your idea of a cradle for 3 months is a great one- DS was in his for 4 months and I will do the same for #2.