Thank you so much for the advice.
I have a very old rollover IRA at Vanguard with about $3,500 in it....can I contribute the $5,500 to that? Or am I better off going with Fidelity?
Yeah, I probably would. I've read that employer sponsored 401(k)'s (not the solo 401k) and IRAs rolled over from them have better liability protection in some states than pure IRAs, but with only $3500 I wouldn't worry about it.
I'd still open the SEP at fidelity and roll it straight into the solo 401k you'll open shortly later so it's all in the same place, but they could help you roll a SEP from vanguard in too, or you could probably just leave a SEP open at the same time as the solo 401k, although you should check on that in case there's anything against having two "employer" plans open at the same time.
As long as having a SEP and solo 401k open at the same time is ok (again, check with whoever you open the solo 401k with), then you could also just leave the SEP open in which case vanguard vs fidelity doesn't really matter. The Fidelity advantage is really just that it accepts rollovers and has lower expense ratios available (not that the vanguard investor class shares are bad by any means).