, you have a lot of questions. Here are some answers.
What type of IRAs can I use to backdoor money into a Roth IRA account without having to pay the 'pro-rata'?-- If the plan has "IRA" in the name the pro-rata rule comes into effect. So Traditional IRAs, SEP IRAs, and SIMPLE IRAs are all a problem when using the "Backdoor Roth" option.
Can I move money from an Individual 401k to a Roth IRA without paying the pro-rata?-- Rolling assets from a 401k to an IRA and then converting to a Roth doesn't in itself involve the pro-rata rule, although the conversion (not the rollover) is taxable. The pro-rata rule come into effect if you have other non-Roth IRAs.
Or is it impossible to entirely avoid paying the pro-rata?-- I'm not sure you understand what pro-rata means. It just means when you convert part of your IRA assets to a Roth, the total assets of all of your non-Roth IRAs (some of which may be after-tax) are taken into account when figuring out the taxes.
I think for traditional IRA/SEP-IRA/SIMPLE IRA, you have to pay a 'pro-rata' to move money into a backdoor Roth IRA?-- Here's an example. Let's say you already have $5K in a Traditional IRA and $10K in a SEP IRA. You want to make a new $5K contribution for this year, but you make too much to deduct it and make too much to contribute to a Roth IRA directly. So you make a $5K non-deductible contribution to a Traditional IRA (either your old one or a new one, it doesn't matter) and immediately convert the $5K to a Roth. If you didn't already have the $5K and $10K, the taxes on the conversion would be negligible. But because of that $15K, most of the conversion is taxable. That's why people who now want to use the "Backdoor Roth" option attempt to get their assets out of non-Roth IRAs, by moving them into their employer's plans.
I read this article and it was saying you can convert a SEP-IRA into an indidvidual 401k and then backdoor it into a Roth IRA. Does pro-rata apply to this type of conversion?-- You are allowed to rollover a SEP IRA to an Individual 401k. That takes the assets out of the IRA. With zero assets in non-Roth IRAs, you can then do the "Backdoor Roth" for NEW contributions.
Unfortunately, Vanguard does not allow SEP-IRA to be converted to a individual 401k. Why is that?-- Every company has its own rules. Maybe they don't want the hassle. Maybe it costs too much. Maybe they'll change their minds later.
Anybody know of any companies that will allow SEP-IRA conversion to an individual 401k?-- Fidelity allows incoming rollovers from SEP IRAs to their Self-Employed 401k plans. See here.
I realize I repeated myself a lot, but maybe one of these explanations will make sense.