Mollies look a lot like female Bettas and male guppies have big tails that can be mistaken for another male Betta. Bettas will usually fight with these type of fish eventually. They rarely show aggression at first if the Betta is new but when he starts establishing his territory he may start bullying the others. Also, you have a total of 10 fish in the ten gallon tank. This is quite overstocked and will lead to aggression not to mention bad water quality. You may want to find a separate 5 gallon tank for the Betta. They are quite happy all alone.
I would consider it as well--I'm not sure what you were told but the betta is going to grow, you know, almost to three inches. The guppies will generally stay kind of small but Phlox is also right about the aggression. It might NOT happen but having a backup situation would be good. You can often get 5 gallon tanks on sale at certain places, at least you can around here. They were a big fad for a while but then people began to discover that they require just as much maintenance with more detail work than a bigger tank and that you can't keep that much in them. However they are perfect for say one betta and perhaps a snail or a 'cleaner fish'.
As I say, it MIGHT work, but I'd strongly advise being ready to move the betta. At the very least those fish will grow and start getting overcrowded, and then aggressive/neurotic behavior might begin.
Oh boy, someone has MTS! HEHE! it happens to the best of us! I'm glad you're going to get another tank for your betta, they really thrive with their own space. Until it's cycled, I hope everything stays calm in the 10 gallon. If you start to have problems, you may be able to use a divider temporarily! Good luck! ~J~