I don't have a lot of experience doing this, and I think the Savage Species book (which I don't have) would help much. I'm going to talk about the Tikmijn and the Damboro here, so if you have no idea what I'm talking about, check my 'Kerakin' thread. I just wanted to jot down a few ideas to get some input from the group.
The Damboro are going to be large size class, and have some sort of impressive modifier to STR and CON. They might also have DR of some sort. They would likely have a major negative to DEX. Because of their size, they would have an larger reach than medium sized characters. They would have a natural 'bull rush' or slam attack. I was thinking that they might need to be one of those races that requires two (or more) levels to reach their full potential.
The Tikmijn are going to be a tiny size class, which in and of itself is a little bit of a challenge. They would have a natural +2 (Small +1, Tiny +2?) to their AC because of their size. They would also get all of the other benefits and weaknesses of their size. They would have a substantial negative to STR and CON, but a major bonus to DEX. They would have a plus to climb and listen checks as well due to their clawed hands and keen ears. In addition, they would use their DEX score for climb checks instead of their STR. They could use their tiny claws as a natural weapon, but they don't do much damage, so they typically use some sort of energy weapon.