Absolutely, it's allowable to get help with projects! But you take on a different type of role, depending on the project: the Project Manager.
First off, decide if you really need someone else coming on, and taking money from you. Yes, they may make you lots more money, but many people want to hire people for the lowest possible rate; can you accept that decent help costs decent money? Would you like being hired at $10/hr, while the project manager (or lead freelancer) is pulling in $100+ /hr? I know I would be pissed off about that...
When dealing with bringing people on, get a contract. No way around it. If you don't use a contract, you have NO protection from them, or the unknown. What happens if they get hit by a bus? What happens if they run away with your idea or proprietary code? Decide on what terms you want, and talk to a lawyer to get a proper contract written up.
Make sure before you start, you have a clear scope of what needs to be done, from start to end. Then, break up the steps. Do not tell them "I need you to make a database". Tell them "I have these requirements for a database, and these constraints. I need xx tables for y, and zz tables for n." You will need to be specific. If you leave the picture too broad, you'll never see the end of the project how you envisioned it.
If you decide to partner with someone on an ongoing basis, then good for you! Hopefully, it means you're busy enough to keep expanding. Make sure you vet your potential partners, as you now must be responsible for this new person. They say on their resume that they know C, but you constantly see
goto statements? Red flag, don't you think?
Good luck! It's a great time when you can bring on help, and get more business and a better reputation! Keep up the good work.