An RMD is a "required minimum distribution", to be a little obvious. It's a very precise amount determined by the IRS. It sounds like one of your concerns is that you satisfy the RMD requirements.
Going step by step:
1. RMD on 401k to IRA transfer:
a) If you are still working at the employer where you had your 401k, then there was no RMD requirement on the 401k.
b) If you terminated work at the employer with the 401k prior to transferring it, then I believe you must take an RMD from that 401k (based on Dec. 31, 2017 balance) before transferring the remainder to the Fidelity IRA. I'm not certain of this; a quick search turned up this piece (without supporting citations):
Since the RMD cannot be rolled over, the plan should first issue one check to the plan participant for the RMD before issuing any checks for a direct rollover. When the check for all the plan funds is issued to the plan participant, he can only roll over any amounts in excess of the RMD
https://www.irahelp.com/slottreport/rmds-must-be-taken-doing-rolloverIf this information is correct, and if you transferred the full amount of the 401k to the IRA, Fidelity should be able to work with you to get the 401k RMD portion distributed to you.
2. Withholding on 401k to IRA transfer:
a) If the check you got from your employer was payable to Fidelity (as trustee or custodian for your IRA), withholding wasn't mandatory. But if you nevertheless elected to have some withheld for taxes, that amount (as well as any RMD, see above) will be taxable to you as 2018 income.
b) If the check you got was payable to you (i.e. you could have cashed the check yourself), then the employer was required to withhold 20% federal tax on the amount of the distribution
above the RMD amount. As above, any money (RMD or taxes withheld) that didn't make it into the IRA will be taxable to you.
You can avoid taxes on the amount withheld by adding this money to your IRA as a 60-day rollover of the 401k. That is, you put back the money within 60 days of receiving the check. Note that you are not allowed to put the 401k RMD into your IRA.
3. 2018 IRA distribution:
a) If you had no traditional IRAs before establishing this one in 2018, you had no IRA RMD for 2018.
b) If you had other traditional IRAs at the end of 2017, then you must compute your RMD for all of those IRAs. You seem to be implying that you had other IRAs, because you talk about taking your RMD distribution (for 2018) by April 2019.
Note the deadline for the first RMD is April 1, not April 15, of the following year. RMD deadlines for subsequent
years are on Dec 31.
https://www.irs.gov/newsroom/many-retirees-face-april-1-deadline-to-take-required-retirement-plan-distributionsIf you have an RMD for 2018 (case (b)), then you must take that RMD amount and set it aside
before doing a Roth conversion. Until you do that, you are not allowed to convert 16% of the IRA into a Roth.
4. Future RMDs and conversions
Each year you will need to compute your IRA RMD based on your December 31 balances and how many
years the IRS says you can expect to live. You must first withdraw that amount from your IRA. After that, you'll be able to do your annual Roth conversions.
Disclaimer - this is not tax advice, just a little information I've picked up as I've gone along. It may or may not apply to you, it may or may not even be accurate information.
