Can I do multiple "stone" types for the same ore?
Instead of this...
"type": "cluster",
"ore": "ore:oreLead",
"stone": "minecraft:stone",

Can I do this? (to add the possibility of the ore generating in sandstone as well)
"type": "cluster",
"ore": "ore:oreLead",
"stone": "minecraft:stone",
"stone": "minecraft:sandstone",

EDIT: That doesn't seem to work.^^

"type": "cluster",
"ore": "ore:oreLead",
"stone": "minecraft:stone, minecraft:sandstone",
This? ^

No, you can, as neither of those constitute valid JSON.

Read the documentation (although since you haven't told me what version you're looking for I've just linked the latest, if you want others, you'll need to find them).

Then consider using an array.