Hi sharperw09.
Welcome to our Mattress Forum. 
The good news is, it’s entirely possible to remedy a too firm mattress. A too soft mattress, however, is nearly always a lost cause.
There are some suggestions in post #2 here that may be helpful with a new mattress that is too firm but if you need some additional softness and pressure relief on your mattress then a topper can certainly be a good solution. The key with a topper is to add “just enough” thickness and softness so that in combination with the mattress it will relieve your pressure points in your most pressure prone position (generally side sleeping) so that there is less alignment risk in your other sleeping positions.
If you do decide to add a topper then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point and guideline to help you choose a topper that has the best chance of success and also includes some links to some good sources for toppers as well.
Also, in regards to sleeping hot, there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range. You may also wish to read more about the Sleeping Microclimate and Temperature Control in post #6 here that talks about all the many factors that are involved in the sleeping temperature of a mattress.
NikkiTMU