Yes, that will work. Ideally, the vessel must be at least 4 to 1 volume of the starter to assure reasonable O2 levels for the starter yeast, so go for it! if the starter is at or near high krausen, you should have no problems. I don't think we know precisely when high krausen is reached, but it would be in a matter of just a few hours, typically. Some here make the SNS starter at the start of the brew day and pitch into their wort when the wort is down to pitching temperature; so the only other thing for you to consider is not "shocking" the yeast with a really cold wort...which you could attemperate by adding some of the cooled wort from the main batch slowly and a little at a time until it gets close to the main batch chilled temp before racking the whole of the main batch onto the starter in your Speidel.