If you connect two such cells with different states of charge in parallel, shouldn't there initially be enormous currents?
Yes. That why you must never do that.
how can you create a common state of charge that allows safe parallel connection?
Ideally: Charge each cell with a single-cell charger up to the maximum voltage (3.6 V for LFP Li-ion cells). Then connect the cells in parallel. All the options you listed are not as good, because the Open Circuit Voltage (OCV) of an LFP L-ion cell is very flat over a wide range of SoC levels.
It is recommend to have same "age" SOH of cell connected in parallel.
That is not a critical requirement. New cells from the same source will have the same SoH. If you're recycling old cells, having the same SoH is the least of your worries.
What is the easiest way to determine batteries with same state of health from a given set?
Determining the SoH of a Li-ion cell is best done with specialized equipment. The easiest way is to buy a Chinese cell tester.
Without it, you can get a rough idea of the SoH of a cell with two parameters: capacity and series DC resistance. Measure the capacity of the cell by fully charging it, then discharging it through a constant load and integrating the current, until the cell reaches a low voltage cutoff. Measure the resistance of the cell by applying a load and measuring the drop in voltage, divide by the current.
But, again, you're overthinking it.
Is the series connection of cells with different SOHs harmless
Yes, as long as they are protected by a properly-installed BMS.
is there a risk of overcharging
No, as long as they are protected by a properly-installed BMS.
when using a charge transfer balancing circuit?
Charge transfer balancing ('active") is pointless in your case. Forget it. Use plain-old bypass balancing ("passive") instead.
Regardless, this question is moot because the idea of balancing cells in series before connecting them in parallel is a needlessly complicated solution.
