Since you wrote about having index mutual funds, then that means you should not use TDAmeritrade which is good when one wants to use ETFs only.
If you want ETFs plus index mutual funds with no fees, then you are left with Fidelity, Schwab, and Vanguard for choices. All of these have house index mutual funds. That is, Fidelity has Fidelity index funds, Schwab has Schwab index funds and Vanguard has Vanguard index funds. Schwab and Vanguard have house ETFs. Fidelity has iShares ETFs.
As a side note WellsTrade has everything free for customers who started a while ago: Vanguard, Schwab, Fidelity, iShares, both index mutual funds and ETFs.
It's all about market timing, uh, I mean rebalancing, uh, I mean opportunistic rebalancing, uh, I mean short-term opportunistic rebalancing due to a short-term change in one's asset allocation.