Hi Mark,
Aristo, LGB, Piko, Trainline45 and AMS (Accucraft) are all pretty much 100% compatible in terms of rail profile. Peco and Tenmille require adaptors as their profiles are different.
Only Aristo has the drilled and screwed joiners, so if you use them you may want to factor in the cost of rail clamps for any new track.
If you're concerned about visual things like sleeper spacing, then keep in mind that most of the other makes are only available with the "euro" narrow-gauge spacing - I think Aristo was the only firm to offer the closer "US" spacing as an option, so do check what you already have.
GRS in Princes Risborough does have some old Aristo track left, I think, if it's just a few new pieces that you want?
Jon.