The only thing to watch is the track radius on curves. Too tight and some locos may have problems. Loco specs variously quote diameter or radius minimums. Of course if it's a layout then large loco overhang on tight curves may hit the scenery.
I fully intend to buy a mess of different brand locos and rolling stock to run on whatever brand track I end up buying and also on say any set-track that may come with a starter set. I just need to keep the scales apart, like a train of 1:32 or 1:29 or 1:24 or 1:22.5 or 1:20.3 each needs to live with its own breed to look slightly authentic. Pity that there's such a mess of scales in Gauge 1, but it all depends on whether you need to model mainline or various narrow gauge lines.
But from what I've seen of most garden railways, you just run anything that you like, it's the fun of playing with trains and not sweating over rivet counting that makes gauge 1 trains so enjoyable.