I'm not an expert, but I can answer a little bit at least.
First things first, to my understanding, no creature with the [MEGABEAST] or [SEMIMEGABEAST] tag will ever actually form a civ. In order to make a cyclops civ, you'll have to remove their [SEMIMEGABEAST] tag first.
In regards to kobolds, couple things: First off, kobolds live in caves, which are not registered on the region maps as civilization areas. Goblins, on the other hand, live in dark towers, which do show up as civilization areas just like towns, forest homes, and dwarven fortresses.
Another difference is in the speaking - kobolds use the [UTTERANCES] tag, so even if they do have [CAN_SPEAK], it doesn't matter. [UTTERANCES] makes the creature speak in meaningless babble, but also does carry a sort of [CAN_SPEAK] function in that it does let the creature have a name.
As far as the difference between megabeasts and semimegabeasts, not entirely sure. Megabeasts are generally more powerful, and less common, but I don't think that's anything to really do with the tag itself. I think semimegabeasts are more likely to settle in caves, while megabeasts are more prone to wander around?
Also, to make megabeasts more likely to find you, you can do a couple things:
1. Make them more survivable. Lots of megabeasts die in worldgen, you can do things like increase their size or use improved bodyplans (especially for the Hydra) to help them live longer.
2. Set worldgen to stop with a lower percentage of megabeasts dead. The default is 80% of megabeasts dead, so there's not all that many left by the time default worldgen finishes.
3. Modify their [FREQUENCY] tag to be higher (ex., a dragon has a default frequency of 5, which is very low when the max is 100)