If you have hard water, yes on the platies. 3 plus one betta fully stocks a ten gallon. :) Personally, I don't like them in smaller than 15 gallons, but many people keep them contentedly enough in a 10.
The shrimp would also be fine, but I *think* some species of shrimp prefer softer water, so make sure you check the water parameters of the species you get. :) I'd start with just 5 shrimp to see how they get along (making sure your boy doesn't eat them, for one thing!) and then get more. :)