So far they've been fine with the betta, although they haven't bred. That's the only reason I don't want to get more just yet unless I need to: because of the risk the betta will change his mind and go eating them.
Everybody knows that you only need 2 to start a colony (one male and one female), but the more you have the better your chances are. With your betta in the mix, it is more than likely that he will be feeding on the babies and your colony will be decimated, especially if your betta decides that he likes the taste of lobster.
Baby Cherry Shrimp are betta food. Maybe with thick, deep mats of Java moss that the Betta cannot penetrate you might get some survivors.
However if you want a good reproductive rate you need a dedicated shrimp tank to get high baby survival rates. And you can start with as little as one berried female. I would get 2 males and 4 females to start with and within 2 months you should have babies. Water parameters are not too important in my experience as long as the tank is cycled and nitrates are kept low. Completely soft water is undesirable, your KH and GH should both be at least 3.
I feed mine a mix of zucchini and spirulina wafers and they are doing fine. The Hikari shrimp food is reportedly quite good but I don't know if it's actually worth the money.