Yes, I believe the Romans' reserved crucifiction for treason, basically, certainly not a minor crime like theft. Mindyou, does the NT actually call them thiefs?I can't remember, if so that definately does not fit historically.
Regarding the slow death, it's true that the Romans, if they were being 'merciful'would brake the victims' legs to speed up what was normally a slow death from suffocation basically.
The NT states that a Roman soldier pierces Jesus'side to speed up death so all could be completed prior to the sabbath. This is supposed to fulfill a "scripture" but it is unclear what scripture it is supposed to refer to. Exodus 12 and Numbers 9 are commandments regarding the pashel lamb and Psalms 34.20/19 talk about in the plural righteous people in general.