In the v5.5 FoglightUpgrade_Field guide page 26 it states that :
A version 5.5.0 Federation Server requires that all Federated Servers be upgraded to version 5.5.0.
What are the side effects of doing this in stages, ie..Federator, then children over the next week or so?
Although it is recommended to upgrade all Federated Server to the same version at the same time, the end result of doing this in stages is that some data may not be in sync until all servers are at the same version.
If the upgrades are going to be prolonged you have the option of stopping the Federator Sync process to reduce server loads.
1. Disable the topology sync in the Federation Server by commenting out all queries
in the federation.config file.
2. Upgrade the Federated Servers.
3. Upgrade the Federation Server.
4. Enable the topology sync, by editing the queries in the federation.config file.
(this is basically what the Federation Field Guide suggests)
At the core level 5.2.5 <-> 5.5.0 federation should be ok, but the situation becomes trickier if cartridge upgrades are involved.