Best not to keep tetras with bettas, as they can be nippy. Yes there are exceptions to the rule, but generally its best to stay away from any nippy fish. Some bettas will not tolerate any company, others will. Depends on the fish. Mine had his fins shredded in the pet shop by tetras and barbs, and now wont tolerate any. I'm currently trying him with some WCMMs, which are meant to be a fairly safe bet, but he has been chasing them constantly for 2 days now, so they may have to come out soon.
They do have some special requirements. They are very territorial so if you wish to keep them with other fish the other fish must go in FIRST, or you can take the betta out and do a rescape before putting them in. They also arent good swimmers, their long fins handicap them. Because of this fast fish can sometimes stress them out, and strong filters are a no-no. It's best to get one with an adjustable flow, again some do okay with stronger flow, but you wont know till you get them. Live plants are fine, any artificial ones should be silk, and no sharp edges on any ornaments, as their fins tear easily. They need access to the top of the tank, as they are labyrinth fish, so tanks like the Fluval Edge with only a small opening which has a filter outlet coming into it are a no-go.
You'll see a lot of info saying they like small spaces because their natural habitat is rice paddies- that isnt strictly true, rice paddies are shallow and crowded but look how big they are: http
/kids.britannica.com/comptons/art-90734/Rice-is-grown-in-terraced-wet-paddy-fields-in-Bali Anyway, bettas can be stressed out by big open spaces, which is partly where the myth that they need small tanks comes from, so in a 30 litre you will want to add more decor, I'd say lots of plants and a coconut cave, to make him feel at home. They like broad-leaved plants, as they can rest on them (they rest a lot as they tire easily from swimming) and build their bubblenests under the ones that reach the surface.
They prefer slightly low PH soft water, but are really adaptable to others. They are a hardy fish, susceptible to stress but easily adaptable, and because they breathe air they dont need an air pump or a lot of water agitation, still water suits them great. They need a meaty diet, so specific betta pellets are best. Most people seem to use Hikari or Attison's pellets.