The ones that don't combine are strictly G1 - they were never later repackaged in G2 packaging so they're not G1.5. The G2 Constructicons can all combine. AFAIK the only differences between them and the green G1 Constructicons are cosmetic - mouldwise they're pretty much the same.

L-R: 1985 G1 Scrapper, 1992 G1 "Scrapper", 1993 G2 Scrapper, Encore Scrapper
Robot mode...


Vehicle mode... (as Demonac said, only the G2 version has the G2 logo)