Wow, you need to slow right down! Why on earth have you been adding more fish when you know there are problems in your tank?! To me thats just plain irresponsible.
A couple of things jump out at me - you have added way too many fish, way too soon. A tank takes about 4-6 weeks to cycle properly and after that time you can GRADUALLY increase your stocking levels. If you go crazy and add a large amount of fish at once the filter won't be able to cope and you'll end up with ammonia and nitrite spikes again. A couple of guppies or half a dozen neons would have been plenty to start it off with.
You also need to look at your total stocking level as it sounds a bit crowded. Can you give us what the dimensions of your tank is and the sizes of your fish at present? That will help to determine whether your tank can cope with that many fish as the stress of over-crowding and problems with poor water quality will also result in fish death.
I would say doing a 10% daily water change will help if there are any high readings of ammonia or nitrite. I would also advise buying your own test kits, as you really need to keep a close eye on at least these two readings, and it will probably work out more expensive for you to keep paying a shop to do it for you. If there are any adverse readings those will be contributing to your ick problem. Stressed fish are more susceptible to disease.
You can add some friendly filter bacteria by using something like Hagen's Cycle or API's Stress-Zyme, this will help speed up the maturation process. You're in the states aren't you? Not sure if you get Tetra products over there but I've been impressed by their disease treatments and would reccommend their Anti Whitespot preparation. I have also heard good things about King British products, but the other medications you mention can knock out the good filter bacteria you need to cultivate, so avoid these if possible.
Finally, do you know what the pH of your water is? As your tetras are doing better than your guppies, it suggests to me it might be acidic, which is fine for tetras but guppies really should be in neutral or alkaline water. They have been bred over so many generations that they would probably adapt but if they are new imports their coping mechanisms might well have been exhausted.
In summary - get test kits, check pH, ammonia, nitrite regularily, add Cycle/Stress-Zyme to speed up filter maturation, add anti-ick treatment, carry out water changes as appropriate to reduce ammonia/nitrite levels, check overall stocking level for over-crowding and reduce numbers/upgrade tank as needed and consider rehoming the guppies if your water turns out acidic. Otherwise set up a separate tank for them with water adjusted to tthe correct pH.
Hope that helps. Let us know how you get on.