1. Well I can't say for certain, but it seems like the case that you have to be initiated in ISKCON specifically. To this day I haven't met any married couples in iskcon who've been initiated by a guru outside of their own ISKCON school. So i've never see an ISKCON devotee married to a Gaudiya Math or Christian devotee etc.
It seems the case that the married couples first started off in brahmachari or brahmacharini ashrams, and then eventually after some training, married persons from their own school. I haven't seen them getting married to outsiders.

2. I don't know if marriage between the two schools is completely out of the question, I can't say that. It just seems like an unspoken 'thing' the iskcon devotees abide by. But I'm only under this impression because I've never met an isckon devotee who just so happened to be married to someone who wasn't part of iskcon. Maybe there are iskcon devotees married to devotees from gaudiya math. It's just I haven't seen it, and most of the devotees in iskcon seem like they'd want to be married within the same family. It certainly be more comfortable.

Of course the central focus or basis of any relationship for a Gaudiya Vaishnava would be Krishna of course, so it seems silly that there would be restrictions on marriage between people of the same Gaudiya vaishnav background. It's not like there's discussion of marriage between a devotee an atheist. But then as dev had said in his comment
"If she/he follows 4 regulative principles and chant 16 rounds as prescribed in ISKCON then go ahead with this relation."
This might be quite an important factor for the person..