File manipulation, stats, metrics - various questions

[von Schultz]

Hi guys! I have three questions i would like to ask!

1.) what is the best way to combine several networks, each contained within one .gephi file?

2.) how can i extract (or make list), say for the top 5-10 nodes for each calculated centrality metric?

3.) can someone explain difference between eigenvector and page rank in the context of Gephi because these metrics are very similar.

Additional explanation regarding #1:
I have 1st degree connections for over 100 Facebook pages and those pages are separated in 10+ categories regarding their type/content.
For each category I've made one .gephi file after I've imported certain number of .graphml files (one page has it's own .graphml) to make one category.
The thing is that I want to analyse each category separately and than everything together.
For some reason if I make one category from several .graphml files nodes/edges are not overwritten (except if they are the same), they are getting unique ID each.
On the other hand if I want to group all made categories from several .gephi files those nodes/edges already have their ID's and those ID's are overwritten.
So the question is if I can get the complete network without combining 100+ .graphml files all over again and use .gephi files instead, BUT to avoid ID's being overwritten...?

Thanks!