Good luck. I would not advise cichlids and gar EVER.
You can hear many claim that this combo has worked for them, but if you take ANY serious gar keepers advise NO, NO & HECK NO!!!
Don't ask if you don't want to hear honest answers.
I noticed the upgrade tank within a "few" years. I utterly despise the dog gone volume gestapo often rearing their ugly tendrils here! If you go into this however, make sure that you understand that the period for this grow-out 90 gallon tank is going to be 2 year absolute max and that given that you are planning on purchasing YOY specimens.
DO NOT DO THIS if you are NOT 100% certain that an upgrade to at least TWICE the current 90G tank will take place for your two YOY's is in your future, and you should be OK.
IMO you have a great combination of a bottom and a mid water column tank predators.