What is the harm in getting the j&j covid vaccine and then a few months later, getting a round of the pfizer vaccine? what about a second dose of the same j&j?

There is a specific schedule for each of these vaccines, and you either get the one JAB of the J&J or the two of the Pfizer. This is not a situation to play "what if" games, as now we have several variants, including Delta. Make a decision on what is available to you, and follow the protocols. Hopefully, you will be appropriately vaccinated.