I need to write a formula which uses both Latin and Greek scripts.
So far I can only make it so that either all characters are Latin, or they're all Greek.
You can simply type Latin letters and Greek letters, if you know how to produce them in the environment. For example, you can switch between English and Greek keyboard layouts. If this sounds inconvenient or inapplicable, you can use the symbol collection called “Catalog”, in the Tools menu. It contains the basic Greek letters both as normal (Greek) and as italic (iGreek). (In a mathematical formula, italic letters should be used for variables even when they are denoted by Greek letters, so you would probably mostly use iGreek.)