Yes you can absolutely and this is the best, and or all females together. if you need more guppies just put a male nd female together. The reason this is best is they grow big and strong. If they are all together they breed like flies and get smaller and smaller as space becomes a premium.
With platys and guppies? You could use zebra danios and small tetras, like neons or cardinals.
Sure, you can keep multiple males together. They might display to each other, but they are not territorial like anabantoids, cichlids, or freshwater "sharks," so they will not seriously fight. What you do want to maintain, though, is a female to male ratio of 2:1 or more. In other words, you should have at least twice as many females as you have males. The reason for this is that the males are constantly trying to court the females, and if their numbers are equal to or greater than the females', then they will exhaust the females and could inadvertently kill them. If you have more females than males, however, the attention gets spread out over many females, and they can handle it. This is the ratio of females to males you find in nature for guppies, and many other fishes, and in other animals as well: about 2:1 or more.
BTW - You should stock Russian tortoises in that ratio, as well, and for the same reason. There should be more females than males.