Note - Some makes are supplied with alternate US style "knuckle" couplers (also used here on 1:1 scale

), along with hook and loop. These will for obvious reasons be featured mostly on US outline products. I started with Aristocraft, USA Trains, Bachman & LGB - all considered "G Scale". These knuckles could be used together satisfactorily, with the exception of Aristocraft. However all would work together with their hook and loop options fitted, along with the other makes mentioned. Be aware you should check that, if buying 2nd hand, the correct option for your use, is fitted/included.
You will start to get into coupler type and height issue compatibility if/when you get into the related, 45 mm gauge compatible, worlds of 16, 15 & 10 mm per foot scales. Examples - UK & US Accucraft, Bachmann, Roundhouse, etc'. But even then there are workarounds if needed. Max