If you having disease related issues with your bettas, adding NEWly purchased live bearers will make matters worse without QTing them.
I find LB fish tend to be worse health-wise than the individually cupped bettas because they're all kept in the SAME tank without the medicated blue water. Genetically I think gups/plats/neons are worse off too.
Keep in mind, these are from my experiences. You may have different view on their instincts. But even with dense jungle like live plant cover & 3x the required filtration, these things wouldn't stop FREAKING out whenever I came near them. 6 females 2 contrasting colorful males.
All of my guppies died about 40 days after I bought them. For some reason, they NEVER bred. Most of them started to develop curved spines & then started to pinecone during their final weeks. A few of them held on for a good 2-3 weeks after showing strange symptoms.
Removing these sick easily spooked fish with the net was a true test of my patience.
Whatever the guppies had, my bettas didn't catch it, so I'm pretty happy about that.
I never liked the skittish guppy behaviors, so I'm glad they're gone.
Oh yeah.... the guppies didn't like the male bettas, the long finned ones usually got nipped.
My female bettas beat down & tore up any guppy that tried to nip.