1. You cannot use a signacantly lower voltage cell with a freshly charged cell. It’s best to use very similarly voltaged cells.
2. You don’t have to let the batteries rest also. Scaru said I don’t have to, and I’ve done this probably 10 times, and my cells are still perefectly fine.
I’d like to just pair up freshly charged cells. But I’d think that having a .1 voltage difference would be the max difference I would feel safe with. Also, I wouldn’t say it’s dangerous to let your cells charge up past 4.20 volts, it’s just a little tiny bit determental to the longevity of the battery. There’s a thread that linked nasa’s testing of 18650’s, where they overcharged, overdischarged and use them in pairs and single cells by themselves. Some of the comments will give you some info. NASA blows up a lot of 18650 batteries I’d prefer if you’d take the cells off the charger and test them once in a while and just take them off at 4.20 volts, but it’s no big worry. A big worry is leaving cells on the charger when a charger just trickle charges after it reaches 4.20 volts. Because alot of cheap chargers don’t monitor the voltage when trickle charging, they just give it a little charge very slowly. But if you leave the cells on the charger for several hours after they are done charging, your cells could get up to 4.25 and higher voltage, which is sorta dangerous then, and bad for the batteries health. But really, don’t worry about the 4.22 volts, if it’s 4.3v i’d worry actually. My charger actually charges to 4.21 volts after letting them settle a little.