Right now, if you have an unlocked phone from another carrier that was 5G or if you purchase a 5G phone from the manufacturer (Apple in this case), it will not work on Cricket 5G at the moment. Only if you purchase a 5G device directly from Cricket (website or store). Of course later we'll allow phones from other companies or manufacturers to be compatible with 5G, including the iPhone 12, but there's no info at the moment.
My point is, I don't recommend to pre-order the iPhone 12 from Apple yet, until it's confirmed on the website from Cricket, that we'll sell it or that other 5G devices that are not from Cricket directly, can work on our 5G network. If you decide to pre-order it, keep in mind you might have an inconvenience of the phone not being compatible with Cricket 5G or network. It happened exactly the same with Samsung Galaxy S20. At first it was not compatible or on sale in Cricket, but later it kicked in.
Regarding the plan, 5G is only be available on the Unlimited More of $60/mo per line (or $90 for 2 lines, $110 for 3 lines, $130 for 4 lines, $160 for 5 lines). Lower plans will only have LTE.