no dx necessary. dd2 was awarded from 6mos because of the level of care required, but her needs were higher.
tbh though, i would hang fire for another month or two whilst the paed sorts out referrals and you know exactly what you are dealing with. easier to fill in the forms (it's a million pages where you have to specify by the minute, day and night, what additional care is required, what additional services/ professionals they access etc etc)
the more evidence you can supply the better. if you only have a dx and have no concrete detail on therapies required, and time it will take, it will be more difficult to prove.
loads of folk go on to have more dcs. for uss, dd2 was dc3, so we were pretty much done anyway. dh hsd the snip fairly soon afterwards because the hospital were still claiming that they didn't know the cause/ might be genetic etc etc. (way after the snip, and on second mri, the independent report said brain damage in classic birth hypoxia pattern). as we already had three, the risk of a genetic complication wasn't a risk we were willing to take. now i would happily have had another one (with a planned cs) but too late.
in actual fact, i think siblings are enormously helpful for kids with sn (particularly pd) certainly it has 'normalised' dd2 and stopped her becoming too precious with the constant stream of adult attention lol. she can argue with the best of them and doesn't expect any special treatment on account of a very small aspect of her make-up. 
nurseries etc can be really beneficial too, in terms of socialising with peer group - very very important, particularly with dcs who are going to lead a pretty normal life and attend mainstream school etc. the only difference really (once they get to school) is that dd might have a couple of appointments a week, and might need some help with mobility. we found that dd2 did get a bit precious at nursery and would speak in terms of going to play with mrs x (her 1-1), so we had to step in really quickly and tweak the balance. mrs x was there to enable dd2 to interact with her peers and access the activities, ie to help promote independence, not to mollycoddle lol.
so, if you want other dcs, go for it. 
you've got plenty of time - in a year or so you'll have a much better idea of ability, and will be much more reassured what the world holds for dd and your family.