What fish do you have right now? Depending on the fish only a few cories would fit. I would get 3 small ones. The more the better. Here I am telling you to get 3 when in my sig it says I have 2 bloodfins and 2 pristellas. I'm going to get more when my 29g is done cycling. Oh, guppies don't need to be in schools so 1 would work.
I second Davy's opinion. Cories should at least be in a group of three. The guppies don't have to be in groups, though. Depending on what those 6 fish are, though, will tell whether or not you can have more fish.
Yes, I think you would be okay adding 3 cories and 2 guppies. Although cories are schooling fish, if you only wanted 2 Im sure they would be fine with just the two of them. I have just 2 and they are great Good luck =)