What would the best gemstones be to make wand magick more effective/powerful?
I believe clear quartz crystal to be very effective, it is an amplifier and can be programed for specific intent, that would be my #1 choice if I had to choose only one.
Can you use more than one?
of course you can, you can do whatever you want here, there is no right or wrong when it comes to what feels right for you, I would recommend putting some serious thought into each stone before adding it to see how it feels for you. I have seen some very nice wands that have 1 gemstone to represent each element and spirit, or, a stone for each chakra.
a wand is a tool, you can have only one that you use for everything or several that are tuned for specific purpose, I myself have several and each has a different feel and purpose, one for healing, another for attraction, etc.
as far a certain words, I don't know about that, I don't use anything specific, I also don't use latin or any other language, I find that the language I understand is the most effective for my work but again, it is a personal choice.
you may also want to look into the energies of the different types of wood that your wand can be made of to further tune your tool