That sounds like a good idea, at least for now. Once you get your big tank cycled you can move them back. This will help them out considerably if your betta tank is cycled. The beneficial bacteria in it will quickly increase to accommodate them.
It wouldn't be a good idea to move the betta into the big tank if there will be much water movement from the filter. Their long flowing fins and tails make it difficult for them to swim against any kind of current and they will soon tire themselves out trying. Premature death will usually follow.
Take your time making a decision. There are so many nice kinds of fish available that if one combination isn't going to work, you will surely find another one that will and will also please you greatly.
