Ok first of all he is still a puppy dogs are not all grown up in the head unti lthey are three so you have a few years yet to get this turned around and going forward. the soner thebetter though as the less he reinforces the bad stuff the better.
Second. I am not a fan of pet coo training they are not concistant across the obard in using postive and they lack a great dea l of training in aggresion.
I suggest you look here at a trainer for postive
Truly Dog Friendly Truly Dog-Friendly Trainers
Scroll down there are a few in Florida.
Third drop the thought that he is jealuos that is a high level of brain funtion and we are still not sure they can do that as yet as it is in the huma npart of the brain that this function occirs and dogs do nto have that part of the brain in thier as developed.
Fourth dogs do not Alpha roll other dogs the one dog shows appesment behaviour to the other. Usually it is the stronger socially knowledgable that will show the belly so the one in fear is less afraid.
Fifth I think that by the sounds of it you have an undersocilaized problem child on your hands that needs a lot of gentle rehab work see the list of trainers above.
Also the nearset vet school shoul dhave a behaviuourist on board and if not Google AVBA and they should have a listing on behaviourist in your area. You do want a PHD packing or APDT packing postive trainer no rough stuff.
Sixth working resouce guarding will help the take it drop it switch for a better item give it back but if he is working guarding with dogs the items need to go away and if he gets them or others then it given where each are seprated.
My girl never gets a bully stick with another dog in the room and if one comes in and I know a stick is about . I find it. Just no reason to leave a trigger around.
If he is guarding the step or blcoking passage put hime to work.. doing something else so that the others can go do their thing.
Seveth start to build hid self confindence teach him silly tricks and get him in agilty work cintrol unleash program and just keep telling him wit htaht clicker hadtreat that he is a good boy he will get it.
I sugget Pat McConnells pamphlet feeling out numbered living in a multi dog house.
JL