Good questions.
Contactor. Theorically it don't have to handle HV. It is between two packs and for example may have only 48v. But I use a tyco ev100.
To choose one contactor there are 3 parameters: amperage, voltage and number of time it can handle this amperage. The more amp. the less number of on/off cycles.
Resistor
When connecting an electronic device with capacitors a resistor is a good design. Capacitor would give huge amp. This process take a few seconds because the capacitor don't store a lot of energy.
But it is not the same problem when paralleling cells.
If connected via a contactor named # 1 with a resistor, amperage will be reduced. This resistor will handle power during a long time until the Delta V become small enough. Then contactor #2 is powered.
The resistor could be hot.
In my Prius there is only one contactor. Max amperage is not too high. Both packs (Nimh and lifepo4) can handle this max amp. Amp being high, time during energy transfer is reduced.
Regen
When braking, before using lithium cells in my Prius voltage could be high, say 270v. After adding lithium cells it is not the same. Because both pack are //, internal resistances are also //. Very little delta V in summer, more in winter of course.
Cells
You have to know the max amp your actual nimh pack have to handle in charging/decharging mode. The max and min voltage.
If your lithium cells don't accep the max regen, using a dc-dc converter is a good way. Efficiency can be high.
Current sensor Hall sensor are interresting. For example allegro one...