If the 2.5isnt cycled it would be worse for them to go in there than the 10 gallon! I don't get why people cycle a tank and while cycling out the fish in a uncycled tank, sorry but it doesn't make any sense!
Don't get guppies till you have the proper housing. The filter is ok, but that goes back to my first point. Why put fish in a uncycled tank while the other tank is cycling, they will be exposed to even more ammonia spikes as the tank is small. Either get them and put them in the 10 gallon or don't get them.