Its sad that you lost a lot of your fish, but honestly, a quarantine tank or getting your fish from a reputable seller would have prevented that problem if it was disease. If a fish you bought there aggressively killed the other fish, then walmart is not to blame for that at all.
As for what to keep with a betta, I wouldnt go with guppies. Even females are territorial and aggressive. The two I've had have been more aggressive than the males I've kept. Guppies, with their bright colors and flowing fins, tend to set off betta aggression. Shrimp are also iffy. Some bettas will leave them alone, some will destroy them all in very short order. It all really depends on the personality of the betta.