I would agree in that you should take any opportunity that comes your way. Model anything for any brand! It’s all about getting professional shots for your portfolio. You can also reach out to photographers who would be willing to do a “test shoot” where everyone donates their time knowing that there is no monetary payout. In return for the experience and your time, you will have professional photographs for your portfolio/Instagram.
I would also suggest networking whenever you can in this industry to get your name & face out there. You want to stand out and have people remember you. Always follow up with a thank you email via LinkenIn or the like. Once you have solid experience, then tell people if a skateboard shoot comes their way, to let you know. You can always sign on with an agency who will get you work; it may not be your dream assignment at first but you have to build solid relationships first. People are willing to help, so if you find someone on LinkedIn, ask if you could talk to them for a brief time or if they are close by, you could ask to meet for coffee and then you’d be in front of them and can ask their advice etc. In my experience, people love talking about themselves and what they do.
In fact, when I wanted to get back into the prop & photo styling field, I researched the industry, professional organizations i was interested in, connected with a couple of local people via Instagram and made so many contacts! I met an amazing stylist from setting up an informational meeting with a producer at New Balance who put me in touch with her. The stylist told me to reach out to the owner of the agency who represented her and after a few months, the agency signed me on as an assistant. Sometimes you have to start at the ground level and work your way up. I had plenty of experience but had been out of the workforce for many years raising my boys, so I was just excited to start somewhere and it has paid off!
AFAIK , there's nothing specific as skateboarding model. One becomes a model and they do the assignments as they come. There could be cases where the company just looks out of photogenic skateboarders for a shoot or professional skateboarders for a shoot. Even then They are more likely to go for a known model who knows skateboarding . For print shoots , one barely even needs to know skateboarding . It is only for motion shoot like Ads or videos where they require a trained person.
You can still reach out to the agencies who generally provide models to such brands and check for requirements. Also ,these companies sometimes do organise auditions when they are looking for some fresh faces. Keep a look out on that.
I'd also suggest you to start with whatever you get. Don't directly just aim for All or nothing mentality. It's ok to take small steps and reach your final goal. All the best!
I think you should take quality pictures when you can and post them to social media using different fashion hashtags and create a portfolio.
You can create an online portfolio of your pictures for free using Wix.com. However, I agree with Prerna - you shouldn't single yourself to one specific brand right now since you're trying to ease into the industry.