That Policy of yours is totally fine. At Zappos, we have a nearly identical Policy:
"All Certified Facilitators are eligible for election for Facilitator for all circles (unless they are the lead link of that circle) even if they are not a role holder in that circle."
Someone has to go through a lot of training to become a "Certified" Facilitator, so it's similar to all the training your Policy requires (but for us, someone must be certified before they can be elected).
There is a lot of grey area in what can and can't be changed Constitutionally with Policies. For the most part, things you can't change explicitly state so. Most other things are ripe for the changing! Below are some examples of things you can't change with Policies, because the Constitution explicitly forbids it:
"2.2.3 Amending the Lead Link Role
A Circle may not add Accountabilities or other functions to its own Lead Link Role, or modify the Role’s Purpose, or remove the Role entirely."
"2.5.3 Amending Elected Roles
... Further, no Circle may amend or remove any Purpose, Domain, Accountabilities, or authorities granted to an Elected Role by this Constitution, nor remove an Elected Role entirely."
"3.2.1 Making Proposals
... A Circle may adopt Policies to further constrain when or how Proposals may be made or processed outside of a Governance Meeting. However, no Policy may limit the right to stop asynchronous processing by escalating to a Governance Meeting."
Hopefully this helps, Lisi!