I bought tickets online but mistakenly only got the palace and not the garden.

Can the garden be viewed from the palace. Is it important to go down into the garden?

Also is the nearby town Heitsing worth spending time exploring?

The gardens were very nice and there was also a zoo. Ypu can probably buy a ticket after you finish the palace. We took the Metro out there and don't recall the town.

Hietzing is a suburbanish district of Vienna:
http://en.wikipedia.org/wiki/Hietzing

The large part of garden is always open to the public. http://www.schoenbrunn.at/en/besuche...ngszeiten.html

There are small sections of the park requiring admissions. None of these required advance purchase with a reserved time slot like the rush-hour like inside the apartment. For example, you can walk up to the Glorietta at no charge. What costs extra is to climb up to the upper deck.

I felt the important choice here was between the Imperial vs the Grand tour. I thought the area covered by the Grand tour beyond the Imperial tour was more interesting and also less stifling due to some visitors finishing the visit at that point.

Does your ticket INCLUDE the carriage collection? That, IMO, is worth the time and effort. Agree that taking the Metro and then the short walk to the entrance is probably the easiest way to visit.

My ticket is for the Grand Tour. Are the fountains part of the free park?

Dukey1, What do you mean by carriage collection?

Carriage collection http://www.kaiserliche-wagenburg.at/en/