the question is a little confusing but i think i got what you're after - when you purchase a unit they may have the option to get a dedicated transport. this allows the unit to start the game mounted on their transport, or come in from reserves in it. but if you feel like it, after you've deployed and the game has started, the vehicle is allowed to pick up other units in your army.
in the case of non-dedicated transports i'm pretty sure any unit that can, may begin the game in it, or come in from reserves in it
like it was dedicated. someone may want to correct me on that one though.
so in the case of your flashgitz, they don't have the option to buy a trukk as a dedicated transport. however, once the game's kicked off they may "borrow" another units trukk. they just can't start the game in it, and can't use it to come from reserves.
they'd be able to do this on a battlewagon however. and along with Looted Wagons its pretty much your only transport option that can be bought for your flashgitz, seeing trukks are only purchased as dedicated transports.
and loading up a battlewagon full of dakka boyz?... well i found out first hand how painful Wayde's battlewagon with ten burnaboyz and CFF mek in it is.
