Yeah the one from vapefiend is the RBT cooling stem so that will fit, not sure about the vaposhop one, and the dreamwood one will definitely NOT fit.
There are standard glass sizes for 19 mm joints, that refers to the width, but the length for a standard size is going to be 26mm (like the Dreamwood stem) or 22mm (which is what RBT Vapes used to be and what Splinter Z still is, but those old RBT stems will not fit regular splinters) so then the regular splinters are a custom 19 mm length as well, hence 19/19. So if it's not an official RBT stem or one made specifically to fit the regular splinter, then it has to be custom cut or sanded down to fit...
Tubo used to be 19/26 But I think now Tetra is also closer to 19/22, but yes different than this regular splinter's 19/19 so those stems will not fit (in fact the only one of those stems that fits for me, is the nub in my SplinterZ and Milaana2)
And the P80 is a fine mod, for the regular splinter, that's what I have my V2 on, running Arctic Fox configured for TCR temp control quite well! I would recommend Arctic Fox over Tubo software (as I am not having much luck trying to use that with a Splinter ZV1 on a G3D)