It is fine for a smaller algae eater like a bristlenose pleco - make sure they don't sell you a common pleco by mistake, as they get much bigger!
If you can get a 20 long, that would be better, as bettas can struggle with taller tanks. A 20 high won't kill your plans, though. It would be fine to stock it with:
- 1 BN pleco
- 1 betta
- 6 x one kind of tetra
- 6 x another type of tetra
Alternatives for the tetras include smaller rasboras (harlequins, mosquito, micro etc), danios (that can be a risk, though), or gentle barbs, such as cherries. Alternatively, livebearers such as platies, guppies or Endlers (my favourite) would do, but only 4 platies or guppies as they have a higher bioload than little tetras.
Just make sure you stay away from nippy fish, such as serpae tetras and tiger barbs. When in doubt, google it and get experiences from hundreds of different people and experts online - don't trust the petshops, who will tell you almost anything in a lot of cases just to make a sale.
Hope this helps. :)