Yes, you can pair older and younger geese. In fact, some people purposefully pair older ganders with younger geese as it's thought to improve fertility. My flock has had a handful of mating shakes ups just in the past year and they all fare through it just fine. They'll have a number more next year and the next and the next and the next. Every time a batch of ganders gets processed a number of geese lose their mates. When a goose or geese are sold, someone loses their mate. If I bring in another goose or geese from outside the farm the whole pecking order is upset, new match ups always emerge as they reestablish the gaggle hierarchy. Geese aren't bonded for life, they don't sulk and die of a broken heart when they lose their mate. They move along just fine, reestablish their place in the group and find a new mate or mates. IME birds of any kind -- chickens, ducks, geese, etc -- will establish inter-species relationships when sufficient same-species relationships aren't available to them, but always quickly revert when those same-species relationships do become available. A lone duck will hang out with and live "happily" as part of a flock of chickens if he doesn't have other ducks with whom to group himself or if he is exiled from an existing group of ducks for whatever reason, but if you add ducks or for whatever reason he is re-accepted into an existing group he'll happily abandon his chickens to join those of his same-species, for example. And though I'm sure exceptions to the rule exist, I'd wager a good sum they're quite rare.
