Oh goodness, I knew it...I'm killing the poor little guys !!
Don't be too harsh on yourself! It probably is the LFS you should be blaming for not telling you about the fish. In future, make sure you do some research before you buy any.
As for a community in a 10 gallon, this is deffinately an option and there's an endless number of fish to choose from.
If you like bettas and guppies, either would work. However, not both at once as bettas and guppies don't usualy get along.
If you want a male betta, a single one with 4-6 pygmy cories or a couple of khulie loaches or a trio of platies (1 male, 2 females) would work. In fact, there's a lot that can go with a betta. The only rules are that it must not be something that will nip (eg: cherry barbs) or is prone to being nipped (eg: hatchetfish) and it must not be related to (gouramies) or look similar to the betta (guppies). Most anything else, provided it stays small, would work. Ask beforehand though to be certain (don't rely on your LFS - ask this forum).
If you'd like mroe than one betta, you can go for females. They have to be in a group of at least 4 to disperse aggression. A couple of kuhlie loaches would work with them but there's not much space for anything else.
Guppies are probably an even better option because they don't limmit you as much as to what you can keep with them. Provided it's small and won't nip, it'll work. A good option (if you want both male and female guppies) is a trio (1 male, 2 females) with a dwarf gourami OR a trio of honey gouramies and either 4 pygmy cories OR 2 kuhlie loaches OR a trio of platies OR 5 or so spotted rasboras. There's all sorts of options. Alternatively, an all-male group of guppies (3-5) would work fine and you won't have to worry about them breeding. You can still mix them with any of the above.
There's plenty of other options - both to mix with the above fish or completely different species. My ideal 10 gallon community would be one with only tiny species:
a trio of sparkling gouramies or 6 spotted rasbora or 4 pygmy cories or a trio of endler's livebearers or a trio of mosquito fish. Any of these could be mixed so that you have a total of around 10 fish in the tank (they all get to about 1") but make sure they are still in the specified group sizes (eg 6 spotted rasboras and 4 pygmy cories).
Are there any fish you particularly have taken a liking to besides guppies/bettas or is there a theme or something you'd like to base your choices around? An idea would be to take back all your fish except the 2 cardinals (just temporarily to keep the tank cycled) and, while you're there, take note of all the small-growing species they have. A list of the common and scientific name for each and note whether you like any in particular. Then, if you post the list, we can try to come up with a combination you'd like.
Alternatively, look around the pictures forum to see if there's something you like the look of. There is bound to be a miniature version you can keep even if what you originaly saw gets too big.