If you are just breaking into G scale with LGB, then stock their track as well. Beware the specialist/niche track systems (anything with less than code 332 rail). There will be some that advocate stocking the product, but, ask the question, 'how much of it are they going to buy?'. To keep the new introductory buyers happy, sell them the same brand of track as the models, get them hooked first, then worry about specialising later.
Nothing worse than getting the 'mix' wrong, and having customers walk away.
Like I said, get 'em hooked, best way is boxed sets, and when they come back for more, expand the model and track range. Beware of the Bachmann offerings, their steel track is strictly indoors only.
In general, almost all brands that use Code 332 rail can be mixed-and-matched without any joining problems, though you need to be aware that the geometry of track pieces does vary between brands. LGB, Piko, Trainline/ML-Train, AMS, Aristocraft will all interconnect and can be used very happily together. The visual appearance of the sleepers (ties) does vary between brands but doesn't affect anything in practical terms.