Yes, you can put different sized models into various sized units. The main exception to this rule is that you can not place characters on monsterous mounts (or indeed monsterous characters unless mentioned otherwise) within units.
The ScarBSB within a Kroxigor unit is a time-honored strategy, so go for it. I do not have my rulebook with me, but unless something changed drastically in 7th edition (I know for a fact mounted units can join infantry units without look out sir penalties), this is a legal ploy.
Do keep in mind, however, that the unit will still have to test for stupidity (the stupidity test occurs before a character can elect to leave the unit), and it cannot charge through skink screens.