Gouramis are particularly problematic because both fish are closely related and both are territorial, so even if the Betta doesn't mind, the Gourami very well might. They also occupy the same top layer of the water. If you insist, dwarf gouramis are supposedly more tolerant, female dwarves in particular (being male the Betta might still have issues, see below). That doesn't necessarily mean it would fit in a 35L tank. The size of the tank is problematic because if either fish had problems, they might have trouble hiding.
Otherwise, good rules of thumb as I understand are to avoid other fish with colorful long fins that the betta might confuse for competition (fancy guppies are a good example), fish that occupy the same region of the water, and fish that might nip the fins of the Betta. That doesn't mean you can't have colorful fish, just make sure they are not at all likely to be confused for another Betta (another reason why gouramis are a bad choice). So I would aim for something reasonably small that stays on the bottom/middle tier, swims quick so it can get away if they Betta gets pushy, and isn't aggressive or nippy.
Bettas are fish with unique personalities. One betta may not tolerate ANY fish around them, on the other hand I've heard of people that successfully kept (DO NOT ATTEMPT THIS!!) two males together without conflict. It all depends on the individual fish.