Sorry I don't know any poetry (except Pam Ayres I am a Witney Blanket, which has no place here being neither from Yorkshire nor Lancs).
Back to the question with due regard for spelling and grammar; a valid type rating permits a pilot to fly that type within its limitations whilst a command is a position of legal responsibility (some might say management position) for the aircraft and its occupants between defined times. I should stop now but I think I'll go on a little.
Irrespective of the time on type, a commander may (and depending on the situation, should) delegate any or all of the flying to their P2 who, in aircraft type rating terms, is equally qualified (companies may place any restrictions on P2 or newly qualified commanders). That said the commander should also be aware that the company, travelling public, authorities and courts have expectations of their abilities and judgement of when to demonstrate those abilities. Any pilot who is not convinced in their own mind that they have the capacity to learn a new type and accept the responsibility of command should refuse the promotion.
During the type rating course and LST the candidate will either demonstrate the required standard and pass or fail to meet the standard and suffer further training or return to their old fleet to gain more experience (there are other scenarios). Assuming a pass the candidate progresses to line training and must demonstrate an ability to carry out the functions of a commander whilst operating the fine machinery. Once this stage is completed and checked the public can be assured that they are in no more danger behind a new captain than they are behind Capt 13000 hours.
Would it be better to do it all on one type; yes but then nobody said it was supposed to be easy and it is your own fault for working at a company with mixed fleets. You really should have discussed this at the interview when they said do you have any questions.