I see, sounds like you're not new to mutual fund investing, and the initial fund minimums won't be a problem. So you could really open Roths with any no-load mutual fund company (American, Vanguard, Fidelity, T. Rowe Price, etc.) or with a discount brokerage that has a fund supermarket available without transaction costs (Schwab, Fidelity).
In other words, if someone is telling you you'd pay a commission every month for buying more shares of a fund, you're looking in the wrong place. Commissions are for stock trades. Both Schwab and Fidelity will give you access to hundreds of funds from dozens of fund families without paying a transaction fee; they'll probably require you to hold the fund at least six months to avoid a fee on the sell, but hopefully you would do that anyway. And because the Roths would then be set up as brokerage accounts you could, if you chose, play around with individual stocks in the same account.
Good luck!