Need pointers on merging pools and upgrading old firmware

I need some guidance on the best plan for pool merge and firmware upgrades.

I have a legacy group that was built by a previous team member. The group has three members and each member has been put into its own pool.

This makes it difficult to safely do firmware upgrades and doesn't allow for performance load balancing.

I would like to merge these three pools into one which I have not done before.

I will also make a maintenance pool that will have no volumes where I can safely temporarily move a member for firmware upgrades without any possible loss of access to volumes during controller restarts which is part of our other group's standard procedures.

Here are the current members and their ancient firmware versions:
san001 - ps6000 - 5.0.2
san002 - ps6000 - 5.0.2
san003 - ps6100 - 5.1.1

My questions are:

1) Will these two model types work together well in the same pool? They all have the same drive sizes and types.

2) Will I be able to merge them and have them function temporarily in the same pool on the above non-matching firmware versions?

3) The server OSs that are accessing the volumes is Citrix XenServer 6.5 and CentOS 6.9 what is the highest version of EqualLogic firmware I can take these members to without causing compatibility issues with the initiators?

4) When I am ready to upgrade them what is the correct process? Can I go right from (5.0.x and 5.1.x) to (6.0.11) to (7.1.12) etc getting all members to the next version before moving on to a higher branch bypassing 5.2.11 and 7.0.10 along the way?

Thanks!