Is it possible to mix rams with different latency
Yes, you can mix DIMMs with different latencies, although you cannot guarantee stability in all cases.
Some people advise buying four DIMMs as a single kit, not two "identical" pairs. Even when you buy two pairs with exactly the same part number, there's no guarantee the memory chips and sub-timings will be exactly the same. Mixing two kits with completely different part numbers just increases the risk.
Depending on how your motherboard interrogates the SPD chip on each DIMM, it may pick a set of timings which are marginally "too fast" for the slowest pair, on the assumption you've fitted four identical DIMMs.
Or it could run the DIMMs with the timings of the slowest pair...