i had one just like it i was told they could go in comunity tanks and in the first night it killed 3 guppies and i couldnt aford a new tank so i had to take it bk
some can live with them but others will attack them so you will just have to watch him
and the reason they attake guppies is the males fins are long and they mistake them for other bettas
Watch his fins the tetra may start ripping them depends on temperment of both the betta and the neons with whom attacks whom you could just be lucky though and have compatible personalities
i just keep a female now she didnt bully the guppies the guppies bullied her so she wedged herself dwn the bk of the filter and her eye half poped out so shes in a 5g tank on her own
I plan to keep mine with guppy's again it depends on the fish and also how you introduce them if you rearrange the tank a bit and remove the fish then introduce them all to the tank together then that supposedly helps with them playing nice.