I know that are compatibility matrix for each FOS release, but how do follow regarding switche's connected as Access Gateway to the fabric? So let's say I have a fabric with FOS 7.4 and connected few access gateways with FOS 6.4 - how would they react if I would upgrade the fabric to 8.1?
FOS 6.4 cannot coesist in a Fabric with FOS 8.x, not matter if the Switch fosconfig is Native or as AG.
FOS 8.x required a minimun of FOS 7.3
To add a bit more info on how you can find the answer to FOS compatibility the Brocade Release Notes are a good source.
You can find the release notes on MyBrocade.com and many are searchable using Google.
Example from FOS v8.1.0c Release Notes page 45:
Supported Products Fabric OS Interoperability4100, 4900, 7500, 7500e, 5000, 200E, 48K Brocade 4012, 4016, 4018, 4020, 4024, 4424No support for interoperability in the same fabric or Access Gateway connection — must use FCR
Brocade 5410, 5480, 5424, 5430, 5431, 5432, 5450, 5460, 5470, NC-5480, VA-40FCv7.3.1 or later
Brocade DCX, DCX-4S, 300, 5100, 5300 v7.3.1 or later Brocade DCX with FS8-18 blade(s), Brocade Encryption Switch v7.3.1 or later
Brocade 7800, DCX and DCX-4S with FCOE10-24 or FX8-24 blades v7.3.1 or later
Brocade 8000 v7.1.2 or later1
Brocade DCX/DCX-4S with FA4-18 blade(s) No support for interoperability in the same fabric — must use FCR2
Brocade DCX 8510-8/DCX 8510-4 FOS v7.3.1 or later
Brocade DCX 8510-8/DCX 8510-4 with FC16-64 blade FOS v7.3.1 or later
Brocade DCX 8510-8 with FCOE10-24 blade FOS v7.3.1 or later
Brocade 6510, 6505, 6530, 7840 FOS v7.3.1 or later
6548, 6547, M6505, 6545, 6546 FOS v7.3.1 or later