A friend has a shop & there is always a small planted tank (or 2) for the CPD's, there is almost always a sale Betta in these tanks as well ...
most Bettas are fine with small fish, while they might spend a bit of effort initially doing some flare & chase, most small fish will easily outmaneuver ornamental Bettas (if you're looking at PK's that's a different scenario, even the most
line bred HMPK's tend to be more focused hunters than their fluffy finned counterparts
) and it's all settled within a few hours or couple days
If you've only one tank or can't rehome Mr Betta, then caution is good - but this statement might apply to many fish species that are considered as CPD tankmates
40litre tank + interesting décor, I'd settle the CPD's then add Mr Betta & have high expectations of success ...
elephant ear/dumbo Bettas also seem pretty low key (though I've not kept these myself), "Giants" though seem to be more predatory & more active (I've no idea on the genetics, most claim mutation but I'm always suspicious of cross breeding having been done in the initial stages) & larger tanks are often recommended