"Strains," "cultivars," "varieties," there's no definite right answer here. I say "cultivar" since other plants go with that term also (i.e apples, peaches, tea).
As for mixing, mixing and matching is encouraged in case one particular variety doesn't quite cut it. It takes a bit to get a feel of the different kava cultivars but once you find you like heady, balanced, or heavy profiles, you can mix and match. They're the same species of plant, after all!
Mixing is great, I do every day. Usually people go from lighter and headier early on to heavier later on, but that’s no reason not to prefer the complete opposite. Keep trying, noting the times you preferred. Everyone has their own kava style, all good.
Lots of people mix and match, but really only after they have a good idea of which kavas they like and which ones they deem as "heavy", "heady" or "balanced". Being new to the kava game I would recommend sticking with one per night for a while until you get the feel of it. That said experimenting with different strains, mixes, different prep methods are all part of the fun of kava.
Man I love Mixing. I normally keep one heady and one heavy cultivar for day or nighttime use. I mix them for balanced effects and to get the full spectrum of kavalactones. Its nice being to alter the effects and taste and dial in what works best for you. Just try finding a few strains you like and combining them until you find the perfect combo.
Drinking a heavy cultivar before a heady will alter the experience of the 2nd session due to the heavy KLs still being active.
Also, if you're buying kava from Vanuatu, Fiji or Tonga then you're most likely already consuming a blend. Most of what people are referring to as "strains" or cultivars are really just blends of random heady/heavy cultivars.