Do not consolidate, you lose points right after a consolidation. If you want a lower payment to afford more house then you need to call FedLoan and change your repayment term but keep that line open because the age of the account is better than consolidating and starting a new line and lowering your age of accounts average.
Yes you can consolidate them and yes it will affect your credit. It will create a new account and close an old account. It may also affect your debt ratios as your interest would be consolidated that is outstanding due to any IBR or IDR you are on, prior to the consolidation as noted above.
This is something that you need to do if you intend to participate in PSLF or if you're disorganized and know you can't manage multiple loans with multiple due dates--you don't say why you were in default so if this is an issue, it's something to consider. Otherwise, if the rates are low, it may be worth it to consider leaving them separate OR, if you're in the private sector and looking to do a quick paydown, getting your scores up and going for a full low-interest consolidation with a bank or lender like SoFi who can get you far better rates than MyFedLoan. If you're doing PSLF, you'll probably HAVE to consolidate. If you've just rehabbed it, you likely have no payments that count as qualifying payments, but if you do, you really want to discuss with them the effect of consolidation. On a new loan, you may lose any qualifying payments and would want to exclude anything already with MyFedLoan from consolidation.
I have 6 different loans with Navieny that I have been paying for a few years. I just finishing rehabbing 6 more loans then they were transferred to Fedloan. I am about to buy a house I really only need to show 1 Student loan payment, so possibly I can afford more house. Can I consolidate these loans into one either with Navient or Fedloan? Also, will it affect my credit?
She absolutely will have to consolidate IF her loan was not a Direct loan. Navient is not only a federal loan servicer. If the loan is not direct, there are no qualifying PSLF payments! See: https://studentaid.ed.gov/sa/repay-loans/forgiveness-cancellation/public-service#eligible-loans
The government websites are very difficult to use, but all of the information is there. Better to read it than go off of what a CSR tells you, 90% of the time, they're wrong. PSLF is a cumbersome nightmare of a program and deliberately so...