How do you fill out your heavy support slots?

Hey everyone last night i purchased 3 leman russ demolisher multi kits i want to know how i should build these, my first thought is:

LR Demo, LR Demo, LR Executioner

The army list will use those three tanks in addition to a manticore so i will have to take two of them in squads to fit everything in the list.

are plasma cannons worth it on the executioner?

which tanks are best in two squads? my first idea is demolishers because they will advance and can use the squad rules to that advantage. I want to protect that executioner because its so expensive!