Can I make a regular attack AND the gore attack gained from this totem, even if my bab isn't high enough to make two attacks ?.
Yes.
If you make a regular attack, the gore will be at -5.
This is the difference between weapon attacks (plus unarmed strike) and natural attacks.
Natural attacks act as either a regular attack during a standard action or something like an "extra" attack during a full attack. There are two types of natural attacks: primary and secondary. Secondary attacks get a -5 penalty to hit, unless you have only one natural attack, in which case it is treated like a primary attack.
Weapon attacks over ride all natural attacks and make them secondary. This can still be preferable to other options like Two weapon fighting since there is no penalty on the weapon attack's hit due tot the natural attacks. Since they have the advantage of being "extra" attacks, they do not get iterative. That means that no matter what bab you have, that natural attack will only ever make one attack a round.
So for example, a level 20 barbarian with the lesser fiend totem and a sword gets 4 sword attacks a round and one gore attack. This might seem like a let down, but most of the time it is best to get as many natural attacks as you have limbs. This is because each attack has your full bab. So while your sword attacks will have bab 20/15/10/5, if you had two claws and a bite for example, you get 20/20/20. The -5 penalty for secondary attacks can also be reduced to a mere -2 with the multiattack feat.