Are they doing a fishless cycle or are you going to cycle with fish? Do they know anything about fish?
If you're going to cycle with fish you'd best start off with zebra danios or platies. Just 3 of either and if you get platies make sure they are all the same sex or you'll get fry while the tank is still cycling (ouch).
I'd say a nice community for a 20 gallon is 6 long-fin zebra danios (if your friends preffer them over the normal ones), 3 platies and 5 bronze cories. The bronze cories are active and hardier than most so they'll make a great bottom-dweller. The platies will add color and activity to the tank as a whole and the zebra danios will add that extra bit of shoaling fun to the top layers. All these fish are hardy but the zebra danios are the hardiest and are best added first, 3 at a time. The platies should be added after that, start with the male and then put in two females. You can add the bronze cories last, 2 at a time. Make sure the seccond group of danios goes in 2 months after the first three (unless a fishless cycle) and the rest of the fish are introduced every two weeks after that.
If these people realy like fish they should like the danios - if they are more of a non-fishy person and are likely to lose interest, get them some guppies which are much more varied in color and also quite hardy (plus they'll breed - added interest) and fill up the tank with more guppies.
Even if they know nothing about fish, putting in the fish like I said, coupled with twice-weekly partial water changes for the first month with de-chlorinated water and then once a week from then on, they should be able to care for the fish. Make sure they know not to over-feed and don't skip water changes.
Help them set up the tank with a suitable filter and heater and let it run for a couple of days to make sure evrything works... get them some test kits and teach them about cycling -
http
/www.fishforums.net/index.php?showtopic=10099 .
On the other hand, they may be quite experienced - if that;s the case, I would suggest 7 harlequin rasboras, 1 dwarf gourami and 4
corydoras leucomelas in a plated tank.
BTW 5 female and 2 male swordtails would be too many and the males would fight. Platies would also be too many. Too many if coupled with the neon tetras that is... and neon tetras are fragile so if this is their first time keeping fish they are not a good choice and would probably discourage them.
