Bettas can be hit or miss--sometimes they are aggressive enough to go after any tankmate, others will be fine in community setups (assuming you are talking about a domestic b Splendens here). The loaches are unlikely to bother the betta. I'd probably start with 4, and then add more, though--they will display much better behavior in the largest group you can provide, and in a 160L, you have room for 8 or 9 quite comfortably, plus the betta and perhaps a small, peaceful barb or danio school.