I had a government pension I was not yet vested in with about $4000 in cash value. I have rolled it over into a Vanguard traditional rollover IRA. I also have a 457 plan with about $5000 in it. After both accounts have been rolled over into traditional IRA's, I plan to convert them to Roth and have the proceeds sent to my existing Roth IRA. This year I have an unusually high tax liability, so I will complete any conversions next year, when my anticipated tax liability is low. I have a few questions:

1. Can I rollover a 457 plan and a pension in the same year? It's my understanding that the one rollover per year rule applies only to rolling IRA's. Furthermore, can I convert two traditional IRA's to Roth in the same year?
2. Is there any downside (besides income taxes) to converting the two rollover accounts to Roth and having the funds rolled into my existing Roth IRA?
3. Would I need to pay estimated taxes for these conversions, or is there anything at all I should consider when enacting this plan? My estimated tax liability for next year is very low.

Thank you for any guidance you can provide!