First of all, what size tank are you planning to use?* I presume you know that the bigger the tank the better off you will be.* The fry will create large amounts of nitrogenous waste products and the more water you have to dilute it the better.*
Have you read much about the process of breeding Bettas and if so what and by whom?* Do you have a set - up to accomidate the female betta after the spawning process has been completed?* She will have to be removed immediately from the male or perhaps even pay with her life.* She will also need exceptionally clean and calm surroundings for a couple of weeks with no tank mates of any kind and calm water movement.* The spawning tank should have no water movement so the bubblenest and surface of the water are not disturbed in any way.* In the first stage of development the prolarvae are extremely sensitive to water movement.* If there is movement in the water it disturbs their tentative hold on the bubblenest.* At this point, they do not have gills or a labyrinth organ to breathe on their own or a mouth to eat, or eyes to see.* All their nutrition comes directily through their tissues from the water.* They are attached to the bubblenest by a sticky substance and can be seen falling from the nest and spiralling back up to the nest to fix themselves back to the nest at least temporarily.* With other fish in the tank it would be impossible to control the movement of water around the bubblenest and since the male betta is the parent that gaurds the nest a fight can ensue if it is disturbed before he is ready to give the job up.
Another thought, if the guppies you are talking about have long fins, the male betta may attack them at any time without provocation whether he is getting ready to spawn or not.* They will fight long-finned fish whether they be bettas or not because they mistake them for another of their own species.
Hope this has helped a little.* I had some more information, but cannot seem to locate it.* You might try
http://www.bettaforums.com I believe they have a whole thread on breeding of bettas.* The pictures are worth the visit while you are there.* *

Yes, I just checked - It's called "The Bubblenest" It is all about breeding bettas and how to get started.
Rose