It’s fine to mix different beddings

. As Rainbow says, Kaytee clean and cost does hold tunnels. Although it might use up quite a lot if pressed firmly down so mixing substrates could work out cheaper as well
Plueschraupe’s set ups are amazing. Oddvon linked them recently as well

. I believe she uses Spruce shavings, which are not easy to get hold of (Spruce is equivalent to hardwood). However Allspan has apparently had issues recently and Hamster welfare no longer recommends it. It is not 100% Spruce. A proportion of it seems to be softwood which, according to their website, isn’t kiln dried. Oddvon found another manufacturer of spruce shavings but not sure if they’re available in the US.
Aspen, hay and Kaytee C&C are fine though. Personally I would also use just Kaytee C&C if it doesn’t work out too expensive. I worry about poky bits with hay and aspen.