Is this 1kV DC all the time (operating) or is it only during transients?
Careful some of those unicorn capacitor ratings are short-term (dielectric test), not continuous operating rating, and they do not include the realities possible on a PCB.
I would design considering safety standards based on IEC 60664-1, which feeds into other standards like 61010, 60730, 60950 etc.
why? It's just a pile of research and experimental data has been done on spacings that are reliable in practice, with aging, to avoid tracking.
This IEC 60664-1 graph which is interesting showing creepage/clearances for high voltages on a PCB, considering pollution degree and PCB laminate (material group).
For pollution degree 2 (non-conductive, dry, no condensation/only temporary) and usual FR-4 laminate CTI <175 (Mat. group IIIb), altitudes under 2,000m, are suggesting minimum creepage of 3mm for 707VRMS/1kVDC, as I read. Just as a number to bounce around, unless you are dealing only with short-term overvoltages, or a safety aspect.
An 0805 (1.2mm pads and lands) roughly 350VDC off the graph.
In a safety function, HV capacitors like X and Y-rated are also subject to mains overvoltages so the spacings are generous with an extra safety-factor applied. They are mainly through-hole parts to achieve the large spacings, as an arc would be disastrous.
I hope this brings to mind some issues to consider for your design.