Doing a 75% 1st mortgage and 15% heloc or heloan would likely give you better cost of funds all around.If this is an investment property, you can do 80% cash out refi using NonQM loans such as DSCR loans, but at a higher cost/rate than if you stick with 75% loan to value. 

I got approved for a HELOAN on my primary property but not sure if I should take the loan to just use as “leverage/reserves” to show the HML and get funded.

And if it's a high amount, would you be better off just using that as a downpayment and get a standard new investment loan?

If that's what you're really after I would start with the basics of what kind of bank loans either residential or commercial are available to you - not saying this is the best option, but it's a good place to start.

If you are moving from your SFR to another property, then you should be able to get a 5% down loan or even 3% if it's FHA.  25% is only if you won't be living in the property in my experience.

Are there any options for lower down payment loans in general or even for college students?

I can say from having taught classes on these loans that your contractor will be the biggest key to your success.

@Mark Sullivan if you are a buy and hold investor these loans work great.

If they can afford to stay, chances are they need to refinance and get the X name off the loan.   

Assuming since you did any FHA loan, this was a househack?