TBH i think you might have trouble fitting a plec into that size tank really, they do like their room, and they get pretty messy. if you're looking at female B. splendens it might be worth looking at other species of betta altogether. most other species you can happily keep in a pair or group and will leave most other fish alone (unless they can fit it in their mouth!) i'm looking into sourcing B. smaragdina or B. prima, but there's plenty of others too, finding them can be tricky, but they shouldn't be any more expensive