Strictly speaking, Races of the Dragon is the “primary” source on most things dragon, including True Dragon and Half-Dragon. Page 69 of Races of the Dragon begins a list of "all True Dragons" in their "half-dragon version." Races of the Dragon explicitly supersedes any information in Draconomicon and the Monster Manual.
To wit: not only is there, according to Races of the Dragon, a half-dragon version of every true dragon, it is explicitly a “version” of the full-blood true dragon. That is, a red half-dragon is a red dragon, just in its half-blood “version.”
Moreover, again according to Races of the Dragon, every half-dragon is a version of a true dragon, and every true dragon has a half-dragon version.
Thus, if your campaign has dragons that you wish to consider True Dragons, Races of the Dragon not only states that they could have a half-dragon version, it states they must, in order to be True Dragons. Of course, any time you introduce homebrew or third-party material, you could easily also introduce a houserule that allows there to be a half-dragon version of lesser dragons, or a type of True Dragon that does not have a half-dragon version.
To address your precise examples, both Ssvaklor and Dragon Turtle are Lesser Dragons, according to Monster Manual, Draconomicon, and Races of the Dragon. According to Races of the Dragon, they therefore cannot have a half-dragon version. Or, put another way, if you gave them a half-dragon version, Races of the Dragon would claim that they are now True Dragons. The Shadow Dragon, on the other hand, is a True Dragon according to both Draconomicon and Races of the Dragon, and both books include rules for a shadow half-dragon.