A properly cycled tank takes weeks... whether with fish or without. Unfortunate, but true. The only shortcut to success is using some mature media stolen from a mature filter. This carries the necessary bacteria needed to deal with the ammonia/nitrite.
Returning the guppies works like this:
First, start your car and let it start warming up.
Next, get a 1 gallon ziptop bag. Fill it 1/3 - 1/2 full of the water from the tank. Put the guppies in there. If you have Prime or Stress Coat, put a drop or two of that in the bag as well. Zip the top ALMOST closed. Insert a straw through the small opening remaining and inflate like a balloon. Then finish zipping it up, trapping as much air in the bag as possible. Place the bag in a small cooler to help maintain the temperature while you drive it back to the store.
Finally, take the cooler and fish into the prewarmed car and drive back to the store. If you have the receipt, good. If not, do it anyway. Tell them that the tank you bought is not sufficient for these fish and you don't want to kill them. (I'd consider mentioning that you were given more than you asked for, which in this case was NOT good.)
You could move the betta to the larger tank. He'd appreciate it. Bettas are very different from guppies in that they can breathe air, and therefore don't require the same elements and filtration as most other fish - although I believe they would appreciate a cycled filter.
I'd wait to put guppies into the 10 gallon tank until it was cycled. But, if you aren't comfortable with returning them, then it would be MUCH better to have the guppies in the 10 gallon tank as the extra volume will help keep the concentration of ammonia much lower. You will still have WEEKS of water changes ahead of you, but the danger will be GREATLY diminished.
You can use Prime or Stress Coat to temporarily detoxify the ammonia. But, this is a short-term solution. Also, products like Ammo-lock, etc. are a waste of money. They will save you the need for the water changes, but you will need to continually add it and it won't help you develop what you TRULY need, which is a healthy and thriving BIOFILTER!
Do you know anyone with a mature tank? If so, beg, plead or steal a small sample from their filter. Even if all you can get is a chance to smear your own filter media on theirs and get the nasty brown stuff on your filter material. That will carry some of the necessary bacteria and jumpstart the whole thing... cutting what could last 6-10 weeks down to a mere week or so, less if you can get a sufficiently large chunk of material!