puppy! congrats, craigslist and freecycle are 2 of my most favoritist places in the world!
the last 90 gallon could be used for ANYTHING, in a 90 you can pretty muhc keep 90% of whats availabel to botht eh fish and reptile world, it realy does depend on what YOU like...you could put together a GORGEOUS cichlid, discus, angel or community tank in a 90 (a saltwater tank would go great too but rather expensive) or go with reptiles...
OR if youd rather not have any pets, set it up with dirst instead of water, hang growlights above it and tada a gorgeous terrerium...
realy want to do something uber cool, look into the diy backgrouns for a half and half tank, half land, half water, and do a biotropoc tank with say newts and fish, or treefrogs and fish (just make sure fishies are too big to be eaten by anything else and too small to eat anything else...
that would make a freeking AWSOME anile tank too, you could turn it into a real jungle a bunch of small lizards and that could look very cool...
or keep it aside for a just in case tank
10 gals is a good starter size, its better to go larger on any tank simply because the more water in the tank the les likely to fluctuate and easier it is to keep your paremeters safe, HOWEVER 10 gals is also a very managable size, the equiptment to run it is CHEAD (overdo the filtreation whever possible, my first 10 came with a hob filter, i hooked up a second Hang on back for 20 gals of filtrration, so i could get away with a touch of overstock.
for smaller tanks i like to go with 1 key specimine (dwarf gourami do well in 10 gals, (blues/threespots and godlds tend to get too big but neon blues and flames as there most commonly called stay smaller, gourami cn be VERY territorial so go with 1 pair of a single. then a nice school of dither fish, something to add activity, zebra danios make a great addtiion becuase i swear there impossible to kill lol! they have no isolated genese in zebras that cause vivid flourescant colors Glofish are simply neopn colored zebras, these unlike many other funny colored fish (ie strawberry tertrs) are NOT tattooed or "painted" ior chemically treated/special food, they are genetically those colors so shouldnt fade, they are active enought to add movment, fast enough to keep out of a gouramis way and small eough you could easily put a nice sized school in there. cardinal and black neon tetras are also nice, regular neons are gorgoues however tend to be VERY fragile...
harlequin rasbora are nice too.
finnish the tank with 3 cory and some ottos for clean up and algea control
lest thats what id do...
there are many DWARF cichlids known as shell dwellers that are fun and cool.
another nice option would be a single pair of german blue rams (or bolivians, gorgous SMALL cichlids with beutifull color and pleasent personalities, you could even add more dither fish to a 10.
good luck, fish keeping is lots of fun, and watching your fish just swmi around in a natrual environment is sooooo nice and relazing...but it can be an expensive and addictive hobby...
right now im already planning my big tank and planning on what to do with the 29 gallon once the big tank is done and ready to go (im thinking im going to do an angel fish/community if i can find the right set up lol. (and an angel that wont eat everyone once full grown lol!)