There's no need to wait to contract them, do that as soon as possible, and then when you get Delia and go to see the nuns with her, she will upgrade the contract for you then.
As far as I know, you'll never be penalized for contracting someone too early, other than being locked into the contracted/property path rather than being in consensual relationships. As long as you eventually meet whatever requirements (presumably just interacting with the girl at the right time(s) and/or enough times) you should be able to upgrade any contracts when the option becomes available.