A lot of cap sets come with alternate sized key caps for keys like alt and shift to fit a range of layouts. Along with novelties of varying sizes. However, if you are truly worried you can also get a true to size print out of your pcb and the key caps to determine exactly which sizes you need for your format but this is mainly for if you are designing your own pcb/ key caps. Edit: just to add on, the only issue you may have is compatibility with your switches, in that case just confirm your key caps are made for the stem of your switch. Most are compatible with Cherry MX Style stems More on reddit.com

Any standard "MX-style" keycap set should fit (not low-profile). The MX-style means the stems on the switches and keycaps have a "+" shape to them. Some switches have a circle or square around the + stem -- (+) or [+] -- but those are fine to use, too, with MX-style keycaps (the shroud around the stem is meant to keep dust out). Do a Google search about the different "profiles" for keycaps so you can see what the different profile styles are. Common profiles are Cherry, OEM, SA, and XDA, though there are others. The Gem80's default keycaps are Nuphy's own style called "mSA", which are a variant of the SA profile, but shorter like a Cherry or OEM keycap. The height, shape and sculpting of the keycaps differ between these profiles, and each key row is slightly different in height and angle (for example, the Z key is much shorter than the F1 key) for ergonomic purposes. XDA profile (and a couple others) are different in that all keys everywhere on the layout are uniform in height and shape/angle. Which to choose is all personal preference. The two most common keycap profiles by far are OEM profile and Cherry profile. The Gem80 is a "TKL" style keyboard, so any keycap set that indicates they work with TKL's should be compatible. It's always best to look at the picture of what's included and match them up to your keyboard, just in case. As for switches, again, any mechanical MX-style switch (not low-profile) will work. There are three main types of switches: linear, tactile and clicky. Tactile switches have a "bump" that you can feel as you're depressing the key that indicates (approximately) the point at which the switch makes electrical contact. Clicky switches are tactile switches that also add a metalic "snap" sound at the same point. Linear switches are smooth all the way down without a tactile bump or audible snap. They all make a little bit of noise when their stems hit the bottom of the housing ("bottom-out"), though some are louder than others and some are quieter (by design). There are a few varieties of "silent" switches, too, that use a little silicone pad at the bottom to dampen out the bottom-out sound. Optical and magnetic switches do not work, FYI. Amazon and other vendors sell switch tester packs that come with one each of several different switches and a dummy keycap & socket so you can try them out without committing to buying a whole set without knowing what you're getting. It's not quite the same as typing on a whole keyboard full, but it'll at least help you narrow the choices down. More on reddit.com

Everybody forgets about the 1800 layout :(

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I'm not sure how much they cost but apple keyboards are pretty solid

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