For rollovers, and what can be rolled over from one thing to another, it is recommended to consult your CPA or tax advisor. Some people might also search on Google for the term “IRS Rollover Chart.”
It is generally understood that everything can be rolled over from one type of account to any other type of account very freely with the one exception of ROTH IRA. When you have ROTH IRA funds, it can only be rolled into another ROTH IRA. Whatever you have in a ROTH IRA if you need to roll it over, what you’re rolling it into will have to also be a ROTH IRA. The Solo 401K has a ROTH component to it, and what that means is you can make contributions to that ROTH component.