If the squad has to be under a certain size in order to take a DT and he is taking more than the squad is allowed and still purchasing it, then he is breaking a rule.
The Imperial guard platoon can be purchased with Chimeras DT for each infantry squad. But upon deployment the ISs can be combined and the combined units are more than the pre-purchased Chimera capacity. At this point the chimeras do not simply dissappear. So they would be deployed on the table empty, and free reign for any other unit to hope in.
I do have a small question about DT and units. I know the rules on a unit buying a DT and IC joining the unit. My question is if the IC buys the DT, can a squad not normally allowed to take a DTbe deployed with him in it, or would they have to embark on turn 1?