Traditional IRA, Taxable IRA, ROTH IRA and 401k distributions

You can withdraw from any account at any time for any reason, there are no restrictions.

I can't tell you about "already taxable" funds in a 401k.  Is that because you have taxable money in a pre-tax 401k, or is it because you have a pre-tax account and a designated Roth account, both at the 401k trustee?

When you reach your RMD beginning age, for purposes of determining the minimum withdrawals required, you consider all traditional IRAs combined as one "individual retirement arrangement".  You calculate the RMD amount from the combined balance, but the actual withdrawal can occur from only one account, or a mixture, as you choose.  Just so long as you withdraw at least the minimum amount from your combined IRAs.

Roth IRAs do not have an RMD and are not included in the RMD calculation for your traditional IRAs.

The pre-tax 401k also has an RMD requirement that is calculated separately from the traditional IRAs and must be satisfied from the 401k. 

Roth 401k accounts have an RMD requirement for 2023 and earlier, but the requirement was eliminated for 2024 and forward.  

I'm not sure what you mean by Taxable IRA; there is no such term defined.  For tax purposes, all of your traditional IRAs are treated in aggregate as if they were all a single traditional IRA with your basis in nondeductible traditional IRA contribution applied to this aggregate.  As you indicated, distributions (other than Qualified Charitable Distributions and HSA Funding Distributions) are a proportionate mix of nontaxable basis and taxable amounts, with the taxable amount calculated on Form 8606.

Distributions from Roth IRAs (in aggregate) and 401(k) accounts are treated separately from each other and separately from your traditional IRAs.  Contact your 401(k) plan administrator to inquire about any plan-specific distribution limitations that might exist.

By "Taxable IRA", I assume you mean a deductible IRA; that is, one in which you took an income deduction for the amount contributed.

You can take tax-free distributions from the Roth IRA independently of anything else.  Easy.

For the IRAs, think:There's no such thing as two different IRAs.  You only have one big IRA, which might be spread over multiple accounts, and which might have come from deductible or non-deductible contributions.  But it's all one big IRA to the IRS.

The distributions are considered to come proportionally from the deductible and non-deductible parts, in proportion to their balances -- regardless of which account you take them from. 

The deductible part is fully taxed; the non-deductible part is tax-free up to the amount you contributed, then fully taxed after that.  In other words, the withdrawal is assumed to come from the contribution first, and the "earnings" are not touched until that's gone.

Just to correct a misconception:  Don't call it "capital gain".  The taxable amounts are taxable as ordinary income, not at the lower capital gain rates.   

I'm sorry, I can't help you with the 401(k) part of your question.  I think I know how this works, but I don't want to give you wrong information.