I would breed in the 10, that way they won't need to be moved as soon.. have some room to grow. Then you will need a grow out.. if you keep the spawn small you can use the 20, but if you have a good amount, over 30 fry.. then I would suggest getting a larger grow out tank.
Sounds like you don't have enough space.. the guppies should be in a 10g+, a 5g is rather small for multiple fish when it comes to the bioload. And you have 3-4 bettas that will need to be homed elsewhere, as you will need the 10 for the spawning and the 20 for grow out (at very minimal you would use a 20 long.. only if you cull heavily).
I would get a 30g+ for grow out, get a 10g for your guppies, use the 20g for your bettas divided up, use the 10g you have for your guppies, and the 5g can be used for additional bettas - if there are one or two you want to keep to breed in the future you can divide it up to grow them out in.
Don't forget all the jars/containers needed for the juveniles, and a way to heat them up.