I don't have any experience, but I'll share what I know.
Use your skeletons (with spears) in two units of 40. At 20 they get +1 attack, and at 30 they get +2 attacks.
Use your zombies as two units of 40, 1 unit of 60, or forgo them (they're very nice with a Corpse Cart). At 20 they +1 to hit/wound, and at 40 they get +2 to hit/wound. Their max is 60, so having 2 units of 40 will likely lose their bonus early and not get it back. A unit of 60 should be able to stay above 40 for a while. A Corpse Cart further enhances their ability to hit.
I've never used Grave Guard, but I've heard that 15 is a good number because less than that and they're vulnerable to easily being wiped off the board and too much more and they're unlikely to get all their attacks. I might recommend two units of 20 and if that seems excessive, drop to 2 units of 15. Great weapons are probably ideal.
If you go with Legion of Nagash Allegiance you can use the grave guard as battleline units (which is the new "Core"), if you decide you don't want to use both skeletons and zombies.
Legions of Nagash Allegiance has some "deathrattle" specific abilities (deathrattle are skeletons, gg, wight kings, and black knights). Wight Kings also have some deathrattle specific abilities. Corpse carts have deadwalker specific abilities (zombies and dire wolves).