The generally accepted method for grouping Lithium Ion cells in a pack is to start with well matched cells (same brand and model and new ideally, and factories will go as far as to buy a massive batch and group similar cells from that in a process called binning) and bring them to the same voltage before ganging them. in addition, balance parallel cells by temporarily connecting them in parallel with a resistor ahead of time to limit current before welding them in parallel.
Weld them with bus strips preferably over using battery holders and strongly over soldering. Welding is preferable as once the batteries are matched they should be kept as a group until end of life.
You have a series arrangement, which means you should indeed use a balancer board. It won't conflict with the protection in the batteries, but if your current was close to the limits of a single 18650 (it's not), you would want to take into account that if one battery dies and removes itself from the circuit, it will double the current through the other battery, increasing it's charge and discharge rate. It's output voltage will change faster than the other sets in the series, so the BMS will have to work harder to balance voltage. The balancer works by wasting the extra power causing voltage mismatch, so efficiency of the entire arrangement will drop, pessimistically similar to if you had changed to a 3S1P arrangement, although it should continue working.
So with those 3 pieces of information: Matching should be done, connections should be permanent and a balancer board is necessary, you should strongly consider buying and already made pack. You may well get better performance for your cost and superior protection, although you may be giving up individually protected cells and the redundancy you're creating.