keep pair intra-distance as close as possible for field cancellation.
No! You need to adhere to a defined distance to get a defined wave impedance. What you describe is a coupled microstrip line.
Fill surround of pair with GND plane and have via near to it to confine the fields, prevent incoming interference and making it.
... in a defined distance to get a defined impedance.
In fact, you don't need a surrounding GND plane on the same layer – practically all field will be between the two differential conductors; what would be good would be a plane below!
The source have 200 ohm differential impedance and the load is 1k differential.
So, that's a high-impedance load and not really a low-impedance source. I'd recommend having two matching networks: one at the source to match the source to the transmission line impedance, and one at the sink.
You could then use an arbitrary transmission line impedance, e.g. the microwave-typical 50 Ω or the 75 Ω. In theory, 200 Ω should work (and would save you the source matching), too, but it might be hard to build using your PCB materials – it depends, can't tell without knowing with what you're working.
And these are contradicting each other can you explain why?
They are not contradicting. A perfect transmission line does not radiate, so your "as close as possible" simply isn't right – yes, close, but not "as close as possible".
Use a specific calculator to calculate the right dimensions for a coupled microstrip line on a PCB substrate of your PCB's thickness, with your PCB's \$\varepsilon\$, and on the frequency you work on.