A business is not permitted to maintain a SEP plan based on the Form 5305-SEP plan agreement and another qualified retirement plan at the same time. To be able to maintain both the SEP plan would have to use a prototype SEP plan agreement other than Form 5303-SEP that explicitly permits both plans to be maintained simultaneously.
There is generally no point to having a SEP plan and a solo 401(k) plan for the same year. The limit on employer contributions is the same to either plan, so whatever would be the employer contribution to the SEP plan (all SEP contributions are employer contributions) could be made to the 401(k) plan instead. The limit on total employer contributions is the same no matter how the employer contributions are split between a SEP plan and a solo 401(k).
titanj, yes you can include your part-time employee and, in fact, you must include the employee if the employee meets the eligibility requirements established in the SEP plan agreement. Your SEP plan must, at a minimum, make employees who have performed services in at least 3 of the last 5 years, are at least age 21, have earned at least $450 during the year and are not covered by a collective bargaining agreement be eligible to participate, but you can adopt less restrictive eligibility requirements, specified in your SEP plan agreement. Your contributions for each participant must be calculated using the same base contribution rate. (The self-employed must use a special adjustment calculation on the base rate. For example, adjusted rate for the self-employed is 20% when the base rate is 25%.)
You cannot have a Solo 401(k) plan if you have eligible employees. You would need a regular 401(k) plan (perhaps a safe harbor 401(k) plan) instead.
