Hi @LORENZO_CONTI,
to answer your questions:
1) theoretically the lenght limitation is given by the cable used (e.g. 100m for CAT5 copper cables)
2) you can use 10Gb/s and multi-mode fiber for ISL (it's the only optical connection we allow for that switch AFAIK), you can confirm which SFP and cables part numbers are needed looking at Hardware Universe.
We officially support only 30m fibre cables when using optical connection (as can be seen in HW Universe).
3) the cluster should theoretically work even with only 1 of the 4 links on the LACP ISL interface, however this might result in extremely poor cluster performance. It is impossible for us to know if removing some links from the ISL will impact the performance of your current setup
Feel free to reply if you have more questions.
Lorenzo