The name of the vehicles and also the name of the slots has changed, so definitely old mods will not work on these The jbeams haven't changed, but since the game engine has changed in these years, they will not deform like in tech demo version
I was about this post where he said about that old mods will not work, the Oldsfullsize is an old mod (from 2016 (that's already pretty old))
Are you thinking that if these mods are active and the oldfullsize is active the oldfullsize wont work? If thats what you think your wrong, they will work fine together. What salim means is if you have an old broken mod active and the one of the front page versions active it wont fix the broken mod.
all you need for the fulsize is the codename oldsfullsize and the separate addon pack if you have both the addon pack works but the addon pack does not work by itself