The SEP IRA is an "employer" contribution only, so it's counted separately from your "employee" contributions that you put into your 401k at work. It's also counted separately from your contributions to your Traditional or Roth IRA. That means you can max out your 401k at work, max out a traditional or Roth IRA, and still put in "employer" contributions into your SEP IRA.,
The main limiting factor, as mentioned above, is the calculation for how much you can put in. It's approximately 20% of your side hustle income, but you'll need to do math to make sure that you've calculated the numbers right.
One thing to consider is whether you plan to utilize a backdoor Roth at any point in the future. If so, you'll want to make sure that you leave all of your pre-tax IRA space empty, including your SEP IRA. Opening up a SEP-IRA is easier, but if thinking long term like that, you might want to consider opting for the Solo 401k. But, you can also just roll over your SEP IRA into a solo 401k later if you find that you need to do that in order to take advantage of backdoor Roths.
I've got a post on how I set up my Solo 401k with Fidelity if that interests you at all. Key thing is fidelity solo 401k is free, so it's just as advantageous as a SEP, I think, and you won't have to worry about backdoor Roth issues later.
IRS website also says this, so you can be comfortable knowing you can contribute to both your regular IRA and SEP IRA at the same time: (source: https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-seps-contributions)
Example 2: Nancy, age 45, is the owner and sole employee of JJ Investment Advisors. Nancy contributes the maximum allowable amount to her SEP-IRA for 2015, or $53,000. Nancy may also make regular, annual IRA contributions to her SEP-IRA, if her SEP-IRA allows this, or contribute to her Roth IRA at XYZ Investment Co. Her total traditional IRA and Roth IRA contributions cannot exceed $5,500 for 2015 and may be made in addition to her SEP contributions.