Checking Yahoo mail is not a problem, however, I would export your Yahoo contacts to a csv file and import it to Gmail, it just makes keeping track and synching your contacts much easier that way. Also, whenever you add a new contact to your phone, don't store them on the phone, store them to the gmail account that way nothing gets lost.
If you depend on having the contacts in your old phone transferred over to the new phone using one of those machines they use at Best Buy and Sprint, just go to the contacts app, export all contacts to your sd card, then import them back into the gmail account (this all works from within the People app just click on Menu and then you will see Import/Export) Now to avoid seeing all those contacts double, you can click on View (also under Menu) and tell the phone to just show your gmail contacts.
But yeah, I have 3 yahoo accounts, 1 Gmail accounts, and 2 other accounts from other providers working on my Evo 4g.
Hope this helps.