wow you deffinately have upgraded with fish hey ...
unfortuntately, 3 bala's in a 75 gal isnt big enough

although bala's love some of their own, they do fine alone as well...I wouldnt get anymore because they are huge gentle giants as they mature...the one you have in your 45 gal, although they grow slow, will need his new tank so his inner organs are not stunted, sooner than later..if you only keep 2 bala's for a 75 gal, that might work..but you wont be able to have more as they wont have room to turn around...they love swimming horizontally back and fourth... ...
once you remove the bala from the 45 gal, you can add some other fish...
as far as angels, its a give or take on if they will eat your neons...some members have ok luck with them, I have not....so thats going to be a gamble ...some members say if the angels grow up with them, they will be fine ..
keep a really close eye on your cycle as that is a ton of fish to add at once to a newly cycled tank....goodluck!