No, strictly speaking.
Let's talk about the parallel connection first. Batteries of different capacities are connected in parallel. According to Ohm's law, a small capacity battery will be exhausted first, while a large capacity battery will continue to discharge. Later, the small capacity battery will be damaged. The damaged battery will not store electricity but will. consume electricity. A faulty battery will slowly drain the power of the batteries connected in parallel.
Regarding the series connection, when you connect 18650 and 26700 batteries in series, you need to make sure that the capacity of the 18650 battery is the same as the capacity of the 26700 battery. Example, the capacity of a single 26700 battery is 5000 mA, and a set of 5 batteries are connected in parallel. This battery set is 5 batteries x 5000mA = 25000mA. You want to connect another set of 1865 batteries0 in series. If the capacity of a single 18650 battery is 2600 mA, then 10 18650 batteries must be connected in parallel to form a set of batteries to be connected in series with the previous 26700 battery.
Finally, there is another problem to note. Since it is a power battery, the 18650 battery cell must also be a power battery cell. The so-called power battery has a significant discharge current. If it is a regular battery, it will leak and break after a few days.
I worked as a technician in a battery factory. There are conditions in the factory, and the voltage must be balanced, the capacitance divided, and the internal resistance measured. can be spot welded together. After assembly, it must be aged for one day (one cycle), and the finished product can only be assembled if it meets the standard.