I agree - another dwarf is out of the question.
The shorter ventral fin is nothing to worry about, provided there's no white or stringy ggrowth at the tip, as it's probably just the result of an old injury. You see, gouramies can easily be nipped orlose part of their ventral fins and, though they do grow them back eventualy, they sometimes never reach full length again.
Also, even if one of your fish were to die, that doesn't mean you have 'an open spot' for another - it simply means something's not right and you have to find out what's wrong (probably the overstocking is linked to it) and fix it.
You have to find out what the other fish you have are - can you post pictures? They should be relatively easy to identify if you could post a few photos (you can upload pics to photobucket.com and then link to them here). Alternatively, try describing them to us and someone may be able to point you in the right direction.
Just to help get you started, take a look at some of these:
guppies (colors vary and these are all males - note the long fins. These are livebearers and do best when kept at a ratio of 2 or more females per male. Alternatively, they do fine in single-sex groups.)
platies (again, the colors vary but there are both males and females in this picture. These do best when kept at a ratio of 2 or more females per male and are livebearers - ie. they give birth to live babies (fry))
neon tetras (cardinal tetras look similar but are all red underneath - instead of half white - and slightly larger. Both cardinal and neon tetras need to be in schools of 6 minnimum.)
clown loaches (these grow very large so hopefuly you don't have any)
swordtails (Colors vary. As you can see in the pic, some of these fish have sword-like extensions on the tail - those are the males. These fish are livebearers like the platies above but males can be somewhat mroe aggressive towards each other and swords grow larger than either platies or guppies. They do better in larger tanks where they ahve plenty of room to swim.)
mollies (These are also lviebearers, similar to guppies. They vary in color and size and aren't realy the best choice for a 10 gallon. I'm not a big fan of these due to their tendency to nip and harass gouramies.)
tiger barbs (though striking and popular, these are a nippy schooling fish that does best in a large tank in a large group and far away from long-finned or slow moving fish)
red tail black shark (these are aggressive and territorial and should be kept singly. They grow to 6" and should be in a 30 gallon minnimum.)
angelfish (much too big for a 10 gallon - that's all you realy need to know about them

)
blue ram (A small cichlid, territorial when breeding. Should be kept in pairs and, idealy, without other territorial fish if it's in such a small tank - ie: no gouramies)
Those are just some of the most popular species and they are what I thought someone might call 'pretty' fish

Of course, also take a look at corydoras catfish, plecostomus catfish and some of the other schooling tetras, rasboras and barbs. They are all very readily available groups and may have caught your sister's eye.