High-Speed PCB Design - Routing on Power Plane Layer?
I assume all your high speed signals are LVDS.

If you are doing signal breakout from the top, then as long as you do not plough through with single ended signals in that area you should be fine.

If there is room on the bottom, that may be a preferable place (so you leave a solid plane). Just ensure you have decent separation rules.

That said, there is no general reason not to use a plane layer for a few single ended signals. Just make sure it is only a few so the copper balance in the stack is not out of whack. Your PCB vendor can help with this.

Give plane preference to the high speed signals, and assuming they are all on the bottom with a solid ground, you should not have any real difficulties as the LVDS signals will all be referenced to ground.